Etude sur les additifs solvants introduits pour

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Etude sur les additifs solvants introduits pour augmenter le rendement de cellules solaires organiques : leurs s´ elections et leurs effets Uyxing Vongsaysy

To cite this version: Uyxing Vongsaysy. Etude sur les additifs solvants introduits pour augmenter le rendement de cellules solaires organiques : leurs s´elections et leurs effets. Polymers. Universit´e de Bordeaux, 2014. English. .

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Logo Université de cotutelle THÈSE EN COTUTELLE PRÉSENTÉE POUR OBTENIR LE GRADE DE

DOCTEUR DE ` ´ ´ THESE EN COTUTELLE PRESENT EE L’UNIVERSITÉ DE BORDEAUX

POUR OBTENIR LE GRADE DE ET DE L’UNIVERSITÉ DE WATERLOO ÉCOLE DOCTORALE DES SCIENCES CHIMIQUES ET DE L’INGENIEUR

DOCTEUR DE SPÉCIALITÉ : CHIMIE PHYSIQUE

´ DE L’ UNIVERSIT E BORDEAUX ET Par Uyxing VONGSAYSY ON PROCESSING USED TO ´ADDITIVES DEINCREASE L’STUDIES UNIVERSIT E DE WATERLOO THE EFFICIENCY OF ORGANIC SOLAR CELLS: SELECTION AND MECHANISTIC EFFECTS Sous la direction de Pr. Laurent SERVANT et de Pr. Hany AZIZ ´ ECOLE DOCTORALE DES SCIENCES CHIMIQUES Soutenue le 25 Novembre 2014 ´ : CHIMIE PHYSIQUE SPECIALITE Membres du jury :

M. SERVANT, Laurent

Professeur, Université de Bordeaux

Directeur de thèse

M. AZIZ, Hany

Professeur, Université de Waterloo

Directeur de thèse

Par Uyxing VONGSAYSY Professeur, Université de Strasbourg

M. HEISER, Thomas

Président et Rapporteur

M. BRISENO, Alejandro

Professeur, Université de Massachussetts Amherst

Rapporteur

M. PAVAGEAU, Bertrand

Représentant industriel, Solvay

Invité

M. WANTZ, Guillaume

Maître de conférences, Institut polytechnique de Invité

Bordeaux STUDIES ON PROCESSING ADDITIVES INTRODUCED TO INCREASE THE EFFICIENCY OF ORGANIC SOLAR CELLS: SELECTION AND MECHANISTIC EFFECTS

sous la direction de M. Laurent SERVANT / M. Hany AZIZ

Soutenue le 25 Novembre 2014 Membres du Jury: M. SERVANT, Laurent M. AZIZ, Hany M. HEISER, Thomas

Professeur, Universit´e de Bordeaux Professeur, Universit´e de Waterloo Professeur, Universit´e de Strasbourg

M. BRISENO, Alejandro M. PAVAGEAU, Bertrand M. WANTZ, Guillaume

Professeur, Universit´e de Massachussetts Repr´esentant industriel, Solvay Maˆıtre de conf´erence, IPB

Directeur de th`ese Directeur de th`ese Rapporteur et Pr´esident Rapporteur Invit´e Invit´e

´ Titre : Etude sur les additifs solvants introduits pour augmenter le rendement de cellules solaires organiques : leurs s´ elections et leurs effets R´esum´e : Les cellules solaires organiques `a h´et´erojonction en volume (BHJ en anglais) font l’objet d’un grand int´erˆet car repr´esentent une source d’´energie bon march´e et renouvelable. Cependant, a` cause des rendements g´en´eralement bas, ce type de cellule peine a` int´egrer le march´e. Afin d’en augmenter le rendement, contrˆoler la morphologie des semi-conducteurs dans la BHJ repr´esente un ´el´ement cl´e. Dans ce contexte, il apparaˆıt, dans la litt´erature, que les additifs solvant permettent de contrˆoler cette morphologie et d’augmenter les rendements. Cette th`ese a pour but de fournir une ´etude compl`ete sur l’utilisation des additifs. Le couple de semi-conducteurs ´etudi´e est le poly(3-hexylthiophene) (P3HT)/[6,6]-ph´enyl-C61 -butanoate de m´ethyle (PC61 BM). Une premi`ere partie pr´esente une m´ethode d´evelopp´ee pour guider la s´election d’additifs parmi une liste de solvants. Cette m´ethode emploie les param`etres de solubilit´e de Hansen des semi-conducteurs. Elle est appliqu´ee au syst`eme P3HT/PC61 BM et r´esulte en l’identification de trois nouveaux additifs performants. Ensuite, des caract´erisations structurales, ´electriques et optiques sont men´ees sur la BHJ et permettent d’identifier les effets des additifs. Les effets de ces additifs se r´ev`elent ˆetre diff´erents en fonction de l’architecture des dispositifs. L’origine de telles diff´erences est corr´el´ee aux variations de mobilit´es des porteurs de charge caus´ees par les additifs. Des tests de photo-stabilit´e ont ´et´e men´es et montrent que les additifs sont capables d’augmenter la stabilit´e des cellules solaires. L’origine de telles am´eliorations est ´etudi´ee. Enfin, l’´etude est ´etendue a` deux autres nouveaux polym`eres semi-conducteurs. Mots cl´ es : dispositif photovolta¨ıque, mat´ eriaux semi-conducteurs, polym` ere

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Title : Studies on processing additives introduced to increase the efficiency of organic solar cells: selection and mechanistic effects Abstract : Polymeric bulk heterojunction (BHJ) organic solar cells (OSCs) have attracted significant interest as a low cost and renewable technology to harvest solar energy. However, their generally low efficiencies are a barrier for their movement into commercial applications. Controlling the BHJ morphology is a key step in the pursuit of higher OSC efficiencies. Processing additives have emerged as effective components for optimizing the BHJ morphology. This thesis provides a comprehensive study on the introduction of additives in the formulation of semiconductors. The semiconductor system studied is based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61 BM). First, a method was developed to guide the selection of additives from a large range of solvents. This method employs the Hansen solubility parameters of the semiconductors and was successfully applied to the P3HT/PC61 BMsystem. It resulted in the identification of three new efficient additives. Next, the mechanistic role of additives in influencing the BHJ morphology is investigated by performing structural, electrical and optical characterizations. Also, the effect of additives on OSC performance was found to depend on the type of the OSC architecture. Such differences were correlated to the variations in charge carrier mobilities caused by the additive. Furthermore, photo-stability tests, performed on different types of OSCs, showed that processing additives can improve the photo-stability. The origin of such improvement is investigated. Finally, the scope of this study is extended to two other donor semiconducting polymers. Keywords : photovoltaic device, semiconducting materials, polymer

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Unit´es de recherche

Institut des Sciences Mol´eculaires (ISM) - UMR 5255 351 cours de la lib´eration 33405 Talence cedex

Laboratoire du Futur (Solvay - LOF) - UMR 5258 178, avenue du Dr Schweitzer F-33608 Pessac

University of Waterloo Department of Electrical and Computer Engineering 200 University Ave. ON N2L 3G1, Canada

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R´ esum´ e en fran¸ cais: Les cellules solaires organiques a` h´et´erojonction en volume (BHJ en anglais) font l’objet d’un int´erˆet grandissant car elles repr´esentent une technologie bon march´e pour extraire l’´energie solaire. Cependant, a` cause des rendements g´en´eralement bas, cette g´en´eration de cellules solaires peine a` int´egrer le march´e. Afin d’augmenter le rendement de ce type de dispositif, plusieurs routes de recherche sont ont ´et´e explor´ees dans la litt´erature, parmi lesquelles figurent la synth`ese de nouveaux mat´eriaux semi-conducteurs, l’optimisation de l’architecture des dispositifs photovolta¨ıques et le contrˆole de la morphologie des semi-conducteurs dans la BHJ. Comme la BHJ est au cœur du m´ecanisme de conversion de photons en porteurs de charge, le contrˆole de sa morphologie a un impact consid´erable sur le rendement du dispositif photovolta¨ıque. Dans la litt´erature, l’introduction d’additifs dans la formulation de semi-conducteurs organiques apparaˆıt comme un ´el´ement cl´e pour contrˆoler - et par cons´equent pour optimiser - la morphologie de la BHJ [169, 170]. Ainsi, de nombreux exemples ont montr´e que l’introduction d’additif peut augmenter de mani`ere significative le rendement des cellules solaires organiques [125, 128]. Malgr´e l’utilisation g´en´eralis´ee des additifs solvant, leurs effets sur la couche active ne sont pas enti`erement compris ni maˆıtris´es et, par cons´equent, leurs s´elections font g´en´eralement l’objet d’exp´eriences de type essai et erreur. Ce type de d´emarche et la m´econnaissance des effets des additifs repr´esentent un obstacle pour l’exploitation compl`ete du potentiel des additifs solvant. Cette th`ese a pour but de constituer une ´etude compl`ete sur l’utilisation des additifs: leurs s´elections, leurs effets sur la BHJ, leurs effets sur les performances du dispositif et leurs effets sur la stabilit´e des dispositifs sont les th´ematiques abord´ees. Le couple de semi-conducteurs majoritairement ´etudi´e dans cette th`ese est le syst`eme poly(3-hexylthiophene) (P3HT)/[6,6]-ph´enyl-C61 -butanoate de m´ethyle (PC61 BM). Une premi`ere partie pr´esente une m´ethode d´evelopp´ee afin de guider la s´election des additifs. L’originalit´e de cette m´ethode r´eside dans l’utilisation de la th´eorie des param`etres de solubilit´e de Hansen pour ´etudier les interactions entre solvants et mat´eriaux semi-conducteurs. Cette th´eorie est g´en´eralement sollicit´ee dans v

l’industrie de la formulation ou de la peinture, son utilisation dans le domaine de l’´electronique organique est, `a ce jour, tr`es limit´ee. Les param`etres de Hansen sont trois param`etres qui d´ecrivent les interactions majeures entre mat´eriaux organiques : les interactions hydrog`enes, les interactions dipˆole-dipˆole et les interactions de London. Comparer les param`etres de Hansen entre un mat´eriau et un solvant donne des informations sur les interactions entre eux et de d´eterminer si elles sont favorables a` la solubilisation ou non. Les param`etres de Hansen sont ici sollicit´es pour ´etudier les interactions entre les mat´eriaux semi-conducteurs organiques (P3HT et PC61 BM) et des additifs ou des solvants. La mise en place de notre m´ethode s’est articul´ee autour de trois axes. Tout d’abord, les param`etres de Hansen du P3HT et du PC61 BM ont ´et´e exp´erimentalement d´etermin´es. La d´etermination de ces param`etres s’est effectu´ee par la r´ealisation de tests de solubilit´e. Ensuite, une ´etude bibliographique a permis d’identifier des additifs efficaces existants. Leurs propri´et´es physico-chimiques et leurs param`etres de Hansen sont mis en corr´elation avec ceux du P3HT et du PC61 BM. Enfin, les r´esultats de ces ´etudes ont ´et´e analys´es afin de d´eterminer une liste de crit`eres (ou r`egles de s´election) que des additifs solvant doivent satisfaire afin d’augmenter le rendement de cellules solaires. Les r`egles de s´election, ainsi d´etermin´ees, sont appliqu´ees `a une liste de 723 solvants afin d’identifier de nouveaux additifs. Parmi cette liste, trois solvants remplissent les crit`eres tout en n’´etant pas canc´erig`enes (pour des raisons de s´ecurit´e) : le phtalate de dim´ethyle, l’o-ac´etylcitrate de tributyl et le 1-cyclohexyl2-pyrrolidinone. Afin de v´erifier la validit´e des r`egles de s´election, les trois additifs sont introduits dans des formulations de P3HT et de PC61 BM destin´ees `a ˆetre int´egr´ees dans des dispositifs. Les r´esultats montrent que les trois additifs sont efficaces et augmentent le rendement de cellules solaires. La plus performante des formulations est obtenue avec le phtalate de dim´ethyle: un rendement de 3.2 % est obtenu (sans recuit thermique), ce qui repr´esente une augmentation de 113 % par rapport a` une cellule solaire de r´ef´erence. L’o-ac´etylcitrate de tributyl et le 1-cyclohexyl-2-pyrrolidinone d´emontrent des augmentations de rendement de 67 % et 93 % respectivement. Ces r´esultats montrent que les additifs ainsi s´electionn´es sont performants et que la m´ethode d´evelopp´ee est efficace pour identifier des additifs solvant. Le chapitre suivant aborde le sujet des effets des additifs solvant sur la morphologie de la couche active dans le but d’expliquer les augmentations de rendement vi

obtenu. Diff´erents types de caract´erisation sur couches minces sont effectu´es : de la diffraction des rayons X (DRX), de la spectroscopie UV-visible et de la microscopie a` force atomique. Aussi, des ´etudes sur la physique des semi-conducteurs et des dispositifs sont conduites par des mesures de mobilit´e de porteurs de charges et par des analyses ´electriques des dispositifs photovolta¨ıques. Les caract´erisations sont men´ees sur films sans additif et avec additif en concentration variant de 0 a` 4 vol % dans la formulation initiale. Afin de v´erifier que les effets observ´es sont universels aux additifs solvant en g´en´eral et non pas sp´ecifiques a` un type d’additif en particulier, les ´etudes sont conduites sur trois additifs avec des structures chimiques diff´erentes. L’analyse des performances photovolta¨ıques montre une diminution de la tension a` circuit ouvert (Vco) avec l’introduction d’additif. Cette diminution de la Vco est attribu´ee a` une augmentation de la cristallinit´e du P3HT qui s’accompagne d’une diminution de sa bande interdite. La diminution de la Vco sugg`ere donc une augmentation de la cristallinit´e du P3HT, r´esultat confirm´e par les mesures de spectres de spectroscopie UV-visible qui montrent un d´eplacement du pic d’absorption du P3HT vers de haute longueur d’onde avec l’ajout d’additif. D’autre part, les mesures des DRX montrent que la taille des cristallites de P3HT diminue avec l’introduction d’additif. La mise en corr´elation de ces r´esultats sugg`ere que l’introduction d’additif provoque la formation de cristallites de P3HT plus petits en taille mais plus nombreux de fa¸con a` ce que la cristallinit´e totale, dans la BHJ, est plus importante que dans un syst`eme sans additif. Ces r´esultats sont coh´erents avec l’´evolution de la mobilit´e des trous mesur´ee dans la couche active et permettent d’expliquer l’´evolution des performances photovolta¨ıques. Afin de comprendre comment l’introduction d’additif induit une telle morphologie, les param`etres de solubilit´e de Hansen sont sollicit´es afin de suivre l’´evolution des interactions entre le syst`eme solvant/additif et les semi-conducteurs organiques pendant le s´echage de la formulation. L’´etude de telles interactions a permis de pr´edire, qualitativement, a` quel moment les cristallites de P3HT sont form´es durant le s´echage et quelles en sont les tailles en fonction de la concentration d’additif dans la formulation initiale. Au terme de ce chapitre, un m´ecanisme expliquant les effets des additifs est propos´e. Dans le chapitre 6, l’effet de l’architecture des cellules solaires sur le mode de fonctionnement des additifs est ´etudi´e. Les cellules solaires peuvent ˆetre fabriqu´ees

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suivant deux architectures: une architecture dite conventionnelle et une architecture invers´ee. La diff´erence entre les deux architectures r´eside dans l’endroit o` u sont collect´es les porteurs de charge: dans une architecture conventionnelle, les ´electrons sont collect´es par l’´electrode du haut et les trous par l’´electrode du bas. Comme l’indique son nom, l’endroit de la collection des porteurs de charge est invers´ee dans une architecture invers´ee. Nous avons montr´e, exp´erimentalement, que ces deux architectures ne sont pas ´equivalentes lorsque des additifs sont introduits dans la formulation de semi-conducteurs: les additifs augmentent le rendement photovolta¨ıque dans une architecture invers´ee mais le diminuent dans une architecture conventionnelle. Les effets des additifs diff`erent donc en fonction de l’architecture employ´ee. Afin de comprendre la provenance de telles diff´erences, les mobilit´es des porteurs de charge sont mesur´ees a` l’aide de transistor `a effet de champ organique. La mobilit´e de saturation est mesur´ee et montre que la mobilit´e des trous augmente avec l’introduction d’additif tandis que celle des ´electrons diminue. Cette diff´erence de mobilit´e est mise en corr´elation avec la distance que les porteurs de charges ont a` parcourir en fonction du type d’architecture. En effet, quelque soit l’architecture utilis´ee, des simulations num´eriques montrent que les excitons sont majoritairement form´es pr`es de l’´electrode transparente, c’est `a dire dans la partie inf´erieure de la couche active. Cela cause une diff´erence dans les distances que les ´electrons et les distances que les trous doivent parcourir avant d’ˆetre collect´es. La mise en corr´elation des variations de mobilit´e avec les distances a` parcourir des porteurs de charge explique les diff´erences observ´ees dans les performances photovolta¨ıques en fonction du type d’architecture. Le chapitre suivant se concentre sur les effets des additifs sur la stabilit´e des cellules solaires. La stabilit´e des cellules solaires organiques est une th´ematique de recherche majeure et il est crucial de s’assurer que les additifs n’impactent pas n´egativement celle-ci. Des ´etudes de spectrom´etrie IR r´eflexion absorption par modulation de la polarisation r´ev`elent des traces d’additif dans la couche active apr`es la formation de celle-ci. Afin de d´eterminer si ces traces d’additif affectent la stabilit´e des cellules solaires, des tests de photo-stabilit´e sont men´es sur des dispositifs avec ou sans additif. Trois types d’additifs sont analys´es. Les tests de photo-stabilit´e consistent en l’illumination continue, sous intensit´e lumineuse de 100 mW.cm-2 , de cellules solaires. Diff´erentes conditions exp´erimentales ont ´et´e mises en place. Les r´esultats montrent que l’introduction d’additif ne d´egrade pas la stabilit´e des viii

dispositifs en comparaison avec un dispositif recuit thermiquement avec un rendement initial ´equivalent. Au contraire, l’introduction d’un type d’additif sp´ecifique contribue mˆeme `a augmenter la photo-stabilit´e des dispositifs. Deux conclusions sont tir´ees de ces exp´eriences : 1. L’introduction d’additif dans la formulation de semi-conducteur ainsi que les traces d’additifs ne d´egradent pas la photo-stabilit´e des dispositifs et 2. L’introduction de l’additif sp´ecifique permet d’am´eliorer consid´erablement la photo-stabilit´e des dispositifs. Afin de comprendre l’origine d’une telle am´elioration, diff´erentes caract´erisations ont ´et´e men´ees. Les r´esultats sugg`erent que la photo-d´egradation affecte primordialement l’interface entre la couche active et l’´electrode m´etallique. Des tests d’adh´esion m´ecanique ont ´et´e effectu´es afin de mesurer l’´energie d’adh´esion entre l’´electrode m´etallique et la couche active. Les r´esultats montrent que l’irradiation continue provoque une diminution de l’´energie d’adh´esion confirmant qu’une photo-d´egradation op`ere `a cette interface. Par ailleurs, les mesures d’adh´esion montrent que, dans le cas o` u l’additif sp´ecifique est utilis´e, le ph´enom`ene de d´egradation interfacial est ralenti, ce qui explique l’am´elioration de la photo-stabilit´e des dispositifs contenant cet additif. Dans le dernier chapitre, les ´etudes sur les additifs sont ´etendues a` diff´erents types de polym`eres semi-conducteurs: un polym`ere amorphe et un polym`ere hautement cristallin. Les diff´erences du mode d’action des additifs en fonction des propri´et´es des polym`eres sont ´etudi´ees.

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Acknowledgements At the end of my engineering school, I had the opportunity to perform this PhD in the field of organic photovoltaics as part of an international program between Canada and France. Back then, I was very excited because I pictured this as a major opportunity in my professional and personal life. Professional, because I was passionate about organic electronics and this PhD was the opportunity to strengthen my knowledge and to contribute scientifically to this innovative field. Personal, because the development of renewable energy is of major importance to me and I was also looking forward to live and work in Canada. Four years later, I am glad to observe that my expectations have been successfully fulfilled and that I gained valuable experiences that were beyond my expectations. I grew up and I learn a lot. This would not have been possible without the large body of people I was lucky and honored to work with and to be friend with. These are the people I want to thank here. First of all, I would like to thank all my supervisors for giving me the great opportunity to perform this PhD and for their guidance throughout these years. I am highly grateful to Bertrand Pavageau for giving me the opportunity to perform this PhD at LOF and for his constant support and his never fading enthusiasm towards my research and my ideas. I would like to address my sincere gratitude to Pr. Laurent Servant for his contribution and his advice in my research but also when I had administrative issues. His availability and his concern for my PhD helped me to carry out my research in good conditions. I am truly grateful to Pr. Hany Aziz for welcoming me in his group. His guidance and his trust in me pushed me to take risks and to do my best. Particularly, during the writing of my thesis, his patience and his advices highly contributed to its quality. To him, I want to express my respect and my sincere gratitude. Sincere thanks go to Dr. Dario Bassani for his support and for fruitful discussions about my research and my manuscripts. I would like to address my sincere gratitude to Dr. Guillaume Wantz and Dr. Lionel Hirsch for welcoming me in the Laboratoire d’int´egration du mat´eriau aux syst`emes (IMS) which is a scientifically great and friendly environment. Special thanks to Guillaume for the discussions; for valuing my work and for always pushing me to be more ambitious. I would like to address my sincere thanks to Pr. Thomas Heiser from the University of Strasbourg, Pr. Alejandro Briseno from the University of Massachusetts and Pr. x

Siva Sivoththaman from the University of Waterloo for accepting to be part of my committee, for reviewing my thesis and for providing valuable comments. I would like to acknowledge Solvay, the Natural Sciences and Engineering Research Council of Canada as well as the Waterloo Institute for Nanotechnology for funding my PhD research. As my thesis was performed in a co-tutelle program, I had the pleasure of being part of four different laboratories. In France, I started my work at LOF Solvay. I thank the director at the time, Patrick Maestro for welcoming me at LOF. I thank Simon Rousseau and Samantha Armisen for sharing their valuable expertise and for their help in the determination of the Hansen Solubility Parameters. I would like to thank all the LOF team for welcoming me and more particularly few buddies for their supports and their friendships: Vincent Mansard (now lost somewhere in the west coast), Julie Angly (for her enthusiasm, her optimism and for the tea), Cyril Vidaillac (for all the valuable conversations), Hongyu (for always being smiley, strong and therefore inspiring), Julien Jolly, Ana Maldonado, Marta Romano, Rawad Tadmouri and many more. In France, I worked in parallel in the Institut des Sciences Mol´eculaires (ISM). In the group of Nano-structures Organiques, I thank Dr. Debdas Ray and Dr. ChihKai Liang who helped with the synthesis of PC61 BM. In the group of Spectroscopie Mol´eculaire, I thank Colette Belin for helping me with the AFM imaging, Thierry Buffeteau and Gwenaelle Lebourdon for their valuable help in the PM-IRRAS experiments on active layer thin films. Still in France, I had the chance to be part of IMS. I would like to thank Dr. Mamatimin Abbas for fruitful discussions and for helping me with transistor fabrication and measurement (despite my stubbornness). I also thank Dr. Sylvain Chambon for always being available for scientific discussions. I truly appreciated his help and his interest in my research. I also want to thank all the members or ELORGA: Pr. Laurence Vignau (for being the first one to trigger my interest in organic electronic), Pr. Pascal Tardy and all the students who brought a enjoyable atmosphere in the lab: Dargie Daribew (my friend), Elodie Destouesse (for her great help when I first arrived in IMS), Maxime Lebail, Lionel Derue, Fr´ ed´ eric Guillain, Gildas Laurans (pour les caramels au beurre sal´e et ta bonne humeur au bad entre autres), L´ eo Peres (malgr´e tes blagues malheureusement sans fin, merci pour ton soutien), Yolande Murat (la plus gentille en 2015 j’esp`ere), Geoffroy Houin (mon xi

grand), William Greenbank, Marcin, Damien Thuau, Pierre Henrie, Th´ er` ese Gorisse, Georgio Mattana, Yu-Tang Tsai and Yan-Fang Chen. In Canada, I was part of the Nanotechnology program of the University of Waterloo in the group of Pr. Hany Aziz. I would like to thank the technical lab managers of the Giga-to-Nanoelectronics lab Richard Barber and Robert Mullins for their role in providing to the students the best working conditions. I would like to address special thanks to Graeme Williams for fruitful discussions and his great help throughout my PhD especially when I first arrived in UW and to Qi Wang for helping me with the adhesion measurements and to achieve a running pace of 5 min/km. Thank you to all the other group members: Afshin Zamani (my coffee buddy), Mike Zhang, Sibi Sutty, Yoshitaka Kajiyama, Anne Bouchauty, Tyler, Thomas, Baolin. They were all unique and inspiring to me in their own way. This PhD project was part of the IDS funmat program. As part of this program, I had the occasions to meet and to discuss with people from a large variety of scientific and cultural backgrounds. It was enriching and fun, thank you guys and especially: Alex Cunha, Marie Asano, Edgar Cao (we are finally in the same city!), Mathilde Champeau, Camillle Legros and An Teaspoon (crazy food and squash buddies) and Annie Cheng (thank you for supporting me through the multiple periods of craziness), Nathacha Kinadjian, Dan (for all the unforgettable cookies), Erin (thank you so much for all the editing, for initiating my passion for yoga and running and for the great time), Medhi, Nhi, Jiang (my bored enemy) and Myl` ene (finalement je t’ai mise l` a, merci pour tout le fun et pour ton support! Je crois en toi pour la suite) and many many more. Last but not least, I would like to thank all my family and Wi for the unconditional support that helped me throughout this journey. Thank you all so much.

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Contents

Abstract

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Acknowledgements

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List of Figures

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List of Tables

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Abbreviations

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Symbols

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1 Introduction to organic solar cells 1.1 General introduction . . . . . . . . . . . . . . . . . . . . . 1.2 Functioning of solar cells . . . . . . . . . . . . . . . . . . . 1.3 Organic semiconducting materials . . . . . . . . . . . . . . 1.3.1 Electronic properties of organic semiconductors . . 1.3.2 Charge carrier generation in organic semiconductors 1.3.3 Charge transport in organic semiconductors . . . . 1.4 Organic solar cells . . . . . . . . . . . . . . . . . . . . . . . 1.4.1 Active layer architectures . . . . . . . . . . . . . . 1.4.2 Formation of BHJ-OSCs . . . . . . . . . . . . . . . 1.4.3 Functioning of BHJ-OSCs . . . . . . . . . . . . . . 1.4.4 Factors influencing BHJ-OSC efficiency . . . . . . . 2 Formulation strategies for controlling BHJ morphology 2.1 Role of solvent . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Effects of post-processing steps . . . . . . . . . . . . . . . 2.3 Use of processing additives in BHJ-OSCs . . . . . . . . . . 2.4 Context and objectives of the thesis . . . . . . . . . . . . .

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1 2 7 9 9 10 11 12 12 14 15 17

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20 21 25 27 30

Contents

3 Experimental details 3.1 Fabrication and characterization of OSCs . . . . . . . . . . . 3.1.1 Materials and substrates . . . . . . . . . . . . . . . . 3.1.2 Active layer formulation . . . . . . . . . . . . . . . . 3.1.3 Substrate cleaning . . . . . . . . . . . . . . . . . . . 3.1.4 Fabrication of OSCs using a conventional architecture 3.1.5 Fabrication of OSCs using an inverted architecture . 3.1.6 J –V characteristics measurements of OSCs . . . . . 3.2 Mobility measurements . . . . . . . . . . . . . . . . . . . . . 3.2.1 Mobility measurement in OTFT configuration . . . . 3.2.2 Mobility measurement in single diode configuration . 3.3 Characterization of the BHJ morphology . . . . . . . . . . . 3.3.1 UV-Vis absorption spectroscopy . . . . . . . . . . . . 3.3.2 X-ray diffraction measurements . . . . . . . . . . . . 3.3.3 Atomic force microscopy . . . . . . . . . . . . . . . . 3.3.4 Infrared absorption spectroscopy . . . . . . . . . . . 3.4 Photo-stability tests . . . . . . . . . . . . . . . . . . . . . .

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33 34 34 35 35 36 36 37 38 38 39 41 41 41 42 42 42

4 Determination of selection rules for processing additives 44 4.1 The Hansen solubility parameters as a method for selecting processing additives . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 4.2 Determination of the HSPs of P3HT and PC61 BM. . . . . . . . . 47 4.3 HSPs of commonly used processing additives . . . . . . . . . . . . 49 4.4 Identification of novel processing additives . . . . . . . . . . . . . 50 4.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 5 The influence of processing additives on the formation of the bulk heterojunction 5.1 Effects of processing additives on the efficiency of OSCs . . . . . . 5.2 Characterizations of films prepared from processing additive . . . 5.2.1 UV-Vis absorption spectroscopy . . . . . . . . . . . . . . . 5.2.2 XRD measurements . . . . . . . . . . . . . . . . . . . . . . 5.3 Hole mobility measurements . . . . . . . . . . . . . . . . . . . . . 5.4 Mechanistic effects of processing additives on the self-assembly of P3HT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

57 58 59 59 60 64 67 71

6 Effect of device architecture on OSCs prepared with additives 72 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 6.2 Electrical performance of OSCs using different architectures . . . 74 6.2.1 OSCs with ODT . . . . . . . . . . . . . . . . . . . . . . . 74 6.2.2 OSCs with DPH . . . . . . . . . . . . . . . . . . . . . . . 76 6.2.3 Conclusions on OSCs with ODT and DPH . . . . . . . . . 79 xiv

Contents

6.3

6.4

Measurements of charge carrier mobility using OTFT configuration 6.3.1 Fabrication of OTFTs . . . . . . . . . . . . . . . . . . . . 6.3.2 Mobility measurements of films prepared from ODT . . . . 6.3.3 Mobility measurements of films prepared from DPH . . . . Origins of additive dependence on OSCs architecture . . . . . . .

80 80 80 82 84

7 The effects of processing additives on the stability of OSCs 87 7.1 Background on the stability issues of OSCs . . . . . . . . . . . . . 88 7.1.1 Stability of organic semiconductors . . . . . . . . . . . . . 88 7.1.2 Stability of OSCs . . . . . . . . . . . . . . . . . . . . . . . 90 7.2 Traces of processing additives in the active layer . . . . . . . . . . 91 7.3 Photo-stability tests on OSCs . . . . . . . . . . . . . . . . . . . . 95 7.3.1 Photo-stability tests on OSCs in air . . . . . . . . . . . . . 95 7.3.2 Photo-stability tests on OSCs in inert atmosphere . . . . . 97 7.4 UV-Vis absorption spectroscopy of light-irradiated active layers . 101 7.5 Photo-stability study on the bottom interface . . . . . . . . . . . 103 7.6 Photo-stability study on the top interface . . . . . . . . . . . . . . 105 7.6.1 Photo-stability tests on OSC active layers . . . . . . . . . 105 7.6.2 Effects of light irradiation on the adhesion of the top electrode107 7.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 8 Studies on new generation donor polymers 8.1 Studies on PDQT . . . . . . . . . . . . . . . . . . . . . . . 8.1.1 Introduction to DPP based copolymers . . . . . . . 8.1.2 Performance of OSCs based on PDQT/PC61 BM . . 8.1.3 Effects of processing additives . . . . . . . . . . . . 8.1.4 Discussions and conclusion . . . . . . . . . . . . . . 8.2 Studies on PCDTBT . . . . . . . . . . . . . . . . . . . . . 8.2.1 Introduction to carbazole based copolymers . . . . 8.2.2 Solubility properties of PCDTBT . . . . . . . . . . 8.2.3 Performance of OSCs based on PCDTBT/PC61 BM. 8.2.4 PCDTBT/PC61 BM/C60 ternary blend. . . . . . . . 8.2.5 Conclusions . . . . . . . . . . . . . . . . . . . . . .

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113 114 114 115 118 122 123 123 124 126 129 136

9 Conclusions and future work 137 9.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 9.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

A Scientific communications

141

Bibliography

143

xv

List of Figures 1.1

Photovoltaic device efficiency within different families of semiconductors. Reproduced from reference [1]. . . . . . . . . . . . . . . . 1.2 Photographs of (a) an OSC printed on paper, reproduced from reference [87] with permission of Wiley and (b) a transparent OSC, reprinted with permission from [34]. Copyright (2012) American Chemical Society . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Building blocks for device fabrication and the three major axes of performance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Example of a typical solar cell characteristic under light illumination and under dark condition. . . . . . . . . . . . . . . . . . . . . 1.5 Equivalent circuit model for a solar cell. . . . . . . . . . . . . . . 1.6 AirMass 1.5 Global solar spectrum. . . . . . . . . . . . . . . . . . 1.7 Chemical structures of: (a) ethylene, (c) polyacetylene and the overlapping of orbitals in (b) ethylene and (d) polyacetylene. . . . 1.8 Simplified schematic of (a) a bi-layer active layer and (b) a BHJactive layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9 Top row: examples of A small molecules and bottom row: example of D polymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.10 Energy diagram in BHJ-OSCs. The figure depicts a situation where the light is absorbed by the D. The photogeneration of the exciton is followed by an electron charge transfer from the LUMO of the D to the LUMO of the A. . . . . . . . . . . . . . . . . . . . 2.1

2.2

3

4 5 7 8 9 10 13 14

16

AFM topography images (2 µm x 2 µm) of as-cast and annealed films of MDMO-PPV-PC61 BM spin-cast from (a, b) chlorobenzene (CB), (c, d) carbon disulfide, (e, f) chloroform (CF), (g, h) pyridine, (i, j) trichloroethylene, (k, l) toluene and (m, n) 1methylpyrrole. Reproduced from reference [57] with permission of Wiley. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 (a) GIXD profiles of P3HT films spin-cast from P3HT/CF solutions containing a range of added acetone; (b) 2θ angle (left axis) of (100) peak and corresponding layer spacing (right axis) as a function of the additional acetone volume ratio. Reprinted with permission from [32]. Copyright (2013) American Chemical Society. 24

xvi

List of Figures

2.3

2.4

2.5

2.6 3.1

3.2 4.1

4.2 4.3 4.4 4.5 4.6 4.7

4.8 5.1 5.2

Effects of thermal annealing on the crystallinity of P3HT depicted by (a) an appearance of P3HT vibronic bands in UV-Vis absorption spectra (b) an increase in diffraction peak of P3HT in XRD pattern. Figures (a) and (b) are respectively reproduced from references [116] with permission of AIP Publishing LLC and [238] with permission of Elsivier. . . . . . . . . . . . . . . . . . . . . . . Proposed model during spin-coating process. Black wire: P3HT polymer chain; big black dots: PC61 BM; blue dots: ODCB molecules; and red dots: ODT molecules. (a–c) correspond to three stages in the spin-coating process when ODCB is the sole solvent; (d–f) correspond to three stages in the spin-casting process when ODT is added in ODCB. Reproduced from reference [234] with permission of Wiley. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Schematic of the role of the processing additive in the self-assembly of BHJ blend materials. Reprinted with permission from [114]. Copyright (2008) American Chemical Society. . . . . . . . . . . . Chemical structures of processing additives. . . . . . . . . . . . . P-type bottom-gate, top-contact OTFT device architecture from a side-view (left) and from a top-view (right). L represents the channel length and W the channel width. . . . . . . . . . . . . . . Hole only device structure. . . . . . . . . . . . . . . . . . . . . . . HSPs diagrams showing the good and the poor solvents resulting from solubility tests and the fitted solubility sphere of the compound under study. . . . . . . . . . . . . . . . . . . . . . . . . . . Solubility spheres of P3HT and PC61 BM. . . . . . . . . . . . . . . Molecular structures of: (a) C-PYR, (b) DPH and (c) TRIB. . . . Positions of C-PYR, DPH and TRIB in the Hansen solubility space with respect to the solubility spheres of P3HT and PC61 BM. UV-Vis absorption spectra of solutions of P3HT in ODCB, C-PYR and TRIB. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Calibration curves obtained from dissolving PC61 BM in CB with different concentrations. . . . . . . . . . . . . . . . . . . . . . . . Solid state UV-Vis absorption spectra for P3HT/PC61 BM blends with varying concentration of processing additives: a) C-PYR, b) DPH and c) TRIB. . . . . . . . . . . . . . . . . . . . . . . . . . . Photovoltaic parameters of OSCs with varying concentration of processing additives: (a) Voc , (b) FF, (c) Jsc and (d) PCE. . . . . Electrical parameters of OSCs with different concentrations for the three processing additives: a) Voc , b) FF, c) Jsc and d) PCE. . Solid state UV-Vis absorption spectra for P3HT/PC61 BM blends with various concentration of a) C-PYR, b) DPH and c) ODT. . .

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26

28

29 30

38 41

47 49 51 52 52 53

54 55 59 60

List of Figures

5.3

XRD patterns of P3HT/PC61 BM fims spin-cast from solutions containing 1.6 vol% of a) C-PYR, b) DPH, c) ODT and d) thermallyannealed film. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Average size of P3HT crystallites as a function of processing additive concentration. . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 XRD patterns of P3HT films spin-cast from solutions containing various concentrations of DPH. . . . . . . . . . . . . . . . . . . . 5.6 (a) Device structure of hole-only device and (b) corresponding energy diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.7 J.L3 as a function of applied voltage for hole-only devices prepared from: a) and d) C-PYR, b) and e) DPH, and c) and f) ODT. Figures a - c ) depict plots for an applied voltage ranging from 0 to 5 V and figures d - f) depict zoomed-in plots in the region 2 to 5 V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.8 Calculated SCLC hole mobility as a function of the concentration of processing additive. . . . . . . . . . . . . . . . . . . . . . . . . 5.9 Evolution of hole mobility and FF as a function of the concentration of processing additive in OSCs prepared with (a) C-PYR, (b) DPH and (c) ODT. . . . . . . . . . . . . . . . . . . . . . . . . . . 5.10 Evolution of the RED with P3HT (in black) and with PC61 BM (in red) as a function of the effective concentration of a) C-PYR, b) DPH and c) ODT. The dotted line at RED of 1 depicts the theoretical RED threshold value separating the regions where the compound is non-soluble and soluble. . . . . . . . . . . . . . . . . 5.11 Schemes representing the BHJ morphology spin-cast from formulations with various concentration of additive. The crystallites of P3HT become more numerous and smaller as the starting concentration or processing additive increases. . . . . . . . . . . . . . . . 6.1 6.2

6.3

6.4

6.5

OSC architecture: (a) inverted and (b) conventional. . . . . . . . Effects of ODT on the electrical parameters in a conventional and in an inverted architecture of OSCs: (a) Voc , (b) Jsc , (c) FF, (d) PCE, (e) Rs and (f) Rsh . . . . . . . . . . . . . . . . . . . . . . . . J -V curves of OSCs with various concentrations of ODT under dark conditions (a) in a conventional configuration and (c) in an inverted configuration and J -V curves under illumination (b) in a conventional configuration and (d) in an inverted configuration. J -V curves of OSCs with various concentrations of DPH under dark conditions (a) in a conventional configuration and (c) in an inverted configuration and J -V curves under illumination (b) in a conventional configuration and (d) in an inverted configuration. Effects of DPH on the electrical parameters in a conventional and in an inverted architecture of OSCs: (a) Voc , (b) Jsc , (c) FF, (d) PCE, (e) Rs and (f) Rsh . . . . . . . . . . . . . . . . . . . . . . . .

xviii

61 62 63 64

65 66

67

69

70 73

75

76

77

78

List of Figures

6.6

Different scenarios of vertical phase separation in BHJ: (a) no vertical phase separation - homogeneous active layer, (b) BHJ with a P3HT-enriched top and a PC61 BM-enriched bottom and (c) BHJ with a P3HT-enriched bottom and a PC61 BM-enriched top. . . . 6.7 Transfer characteristics of OTFTs prepared with various concentrations of ODT in: (a) p-type OTFTs and (b) n-type OTFTs. . . 6.8 (a) Mobility values of OTFTs prepared with various concentrations of ODT as a function of VGS -Vth : the left side of the graph depicts hole mobility and the right side the electron mobility. (b) Electron and hole mobilities at VGS -Vth of +2 V and -2 V respectively, as a function of ODT concentration. . . . . . . . . . . . . . 6.9 Transfer characteristics of OTFT prepared with various concentrations of DPH in: (a) p-type OTFTs and (b) n-type OTFTs. . . 6.10 (a) Mobility values of OTFTs prepared with various concentrations of DPH as a function of VGS -Vth : the left side of the graph depicts hole mobility and the right side the electron mobility. (b) Electron and hole mobilities at VGS -Vth of +2 V and -2 V respectively, as a function of DPH concentration. . . . . . . . . . . . . . 6.11 A schematic description of hole and electron transport in OSC without processing additive and subsequent preferential architecture. 6.12 A schematic description of hole and electron transport in OSC prepared with 2.4 vol% of additive and subsequent preferential architecture. OSCs with 2.4 vol% depict the case where hole mobility is higher than electron mobility. . . . . . . . . . . . . . . . . 7.1

7.2

7.3 7.4

7.5

(a) Normalized UV-Vis absorption (at 500 nm) of MDMO-PPV (4), MDMO-PPV/PC61 BM ( ) and normalized UV-Vis absorption (at 520 nm) of P3HT (•) and P3HT/PC61 BM () samples during photo-oxidation. (b) Normalized UV-Vis absorption of MDMO-PPV/PC61 BM (at 500 nm ( )) and P3HT/PC61 BM (at 520 nm ()) samples during photolysis. Reproduced from reference [180] with permission of Elsivier. . . . . . . . . . . . . . . . . Optical microscopy images of P3HT/PC61 BM layers before and after being thermally annealed at 150o C for 5 hours or 24 hours. Reproduced from reference [222] with permission of Wiley. . . . . Scheme of the procedure for the detection of processing additive in P3HT/PC61 BM films. . . . . . . . . . . . . . . . . . . . . . . . (a) ATR spectrum of C-PYR, (b) Zoomed-in PM-IRRAS spectra of P3HT/PC61 BM films with and without C-PYR (region 1900 1600 cm−1 ). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (a) ATR spectrum of DPH, (b) Zoomed-in PM-IRRAS spectra of P3HT/PC61 BM films with and without DPH (region 1820 - 1700 cm−1 ). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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93

List of Figures

7.6

7.7 7.8

7.9

7.10

7.11

7.12 7.13 7.14 7.15

7.16

7.17

7.18

7.19 8.1

(a) ATR spectrum of TRIB, (b) Zoomed-in spectra of the region 1800 - 1700 cm−1 of PM-IRRAS spectra of P3HT/PC61 BM films with and without TRIB. . . . . . . . . . . . . . . . . . . . . . . . 94 (a) ATR spectrum of ODT, (b) PM-IRRAS spectra of P3HT/PC61 BM films with and without ODT. . . . . . . . . . . . . . . . . . . . . 95 Normalized electrical parameters of OSCs subjected to light irradiation in air: (a) Jsc , (b) Voc , (c) FF, (d) PCE, (e) Rs and (f) Rsh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 Normalized electrical parameters of OSCs subjected to light irradiation in inert atmosphere: (a) Jsc , (b) Voc , (c) FF, (d) PCE, (e) Rs and (f) Rsh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 UV-Vis absorption of P3HT/PC61 BM films before and after light irradiation for 60 hours in air for (a) Thermally annealed, (b) CPYR, (c) DPH and (d) ODT-treated active layers. . . . . . . . . . 101 UV-Vis absorption spectra of P3HT/PC61 BM films before and after light irradiation for 60 hours in inert atmosphere for (a) Thermally annealed, (b) C-PYR, (c) DPH and (d) ODT-treated active layers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 Scheme of photo-stability tests on OSC with a C60 buffer interlayer.103 Energy levels of OSCs containing C60 as an interlayer between ZnO and the active layer. . . . . . . . . . . . . . . . . . . . . . . 104 Effects of C60 thickness on the PCE of OSCs. . . . . . . . . . . . 104 Evolution of normalized (a) Voc and (b) PCE as a function of irradiation time for OSCs without (solid line) and with (dashed line) a C60 interlayer. . . . . . . . . . . . . . . . . . . . . . . . . . 105 Comparison of the normalized (a) Voc and (b) PCE after 40 hours of light irradiation on OSCs without top electrode or with top electrode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 (a) Schematic drawing of a four-point bend adhesion sample stack. A load P/2 is applied on each side of the sample. (b) A typical load versus displacement characteristic. Figure (b) is reproduced from reference [220] with permission of AIL Publishing LLC. . . . 108 Load versus displacement characteristics for fresh and irradiated samples for (a) samples with ODT and (b) thermally annealed samples. On (a) the measured force loss (∆N) is showed. The plots of the irradiated samples were manually vertically down shifted for the clarity of the Figures. . . . . . . . . . . . . . . . . . . . . . . 109 AFM topography (1 µm x 1 µm) images of active layers films: (a) thermally annealed and (b) ODT-treated. . . . . . . . . . . . . . 111 On the left: DPP-based conjugated polymers where R is a substituent, Donor 1 and Donor 2 are electron donating building blocks. On the right, examples of electron donating building blocks are depicted. Adapted from reference [121] with permission of the Royal Society of Chemistry . . . . . . . . . . . . . . . 114 xx

List of Figures

8.2 8.3 8.4 8.5 8.6 8.7

8.8 8.9

8.10 8.11 8.12 8.13 8.14 8.15 8.16 8.17 8.18 8.19 8.20 8.21

8.22

Chemical structure of PDQT. . . . . . . . . . . . . . . . . . . . . 115 Solid-state UV-Vis spectrum of PDQT/PC61 BM in a 1/3 ratio. . 115 Energy levels of PDQT/PC61 BM - OSC in an inverted architecture.116 Solubility spheres of PC61 BM and PDQT. . . . . . . . . . . . . . 116 XRD patterns of: (a) pure PDQT and (b) PDQT/PC61 BM blends in various D/A ratios. . . . . . . . . . . . . . . . . . . . . . . . . 118 AFM images of PDQT/ PC61 BM in various D/A ratios: (a-d) phase images, (e-h) topography images: (a) and (e) ratio pure PDQT, (b) and (f) ratio 2/1, (c) and (g) ratio 1/2, (d) and (h) ratio 1/4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 Electrical performance of PDQT/PC61 BM-OSCs as a function of additive concentration: (a) Jsc , (b) Voc , (c) FF and (d) PCE. . . . 120 AFM images of PDQT/PC61 BM in a 1/3 ratio prepared with various concentrations of additive: (a-c) topography images, (d-f) phase images: (a) and (d) no additive (b) and (e) 5 vol%, (c) and (f) 11 vol%. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 Chemical structures of: (a) a carbazole unit and (B) PCDTBT. . 123 Energy levels of PCDTBT/PC61 BM-OSCs in an inverted architecture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 Solubility spheres of PCDTBT (in blue) and PC61 BM (in red) in the Hansen solubility space. . . . . . . . . . . . . . . . . . . . . . 125 Photovoltaic parameters of OSCs as a function of the thickness for various D/A ratios: (a) Jsc , (b) Voc , (c) FF and (d) PCE. . . . 127 Hansen solubility spheres of PCDTBT and PC61 BM and the following solvents: DIO (black), ODT (yellow). . . . . . . . . . . . . 127 Solid-state UV-Vis absorption spectra of PCDTBT/PC61 BM films (ratio 1/3) prepared without or with DIO. . . . . . . . . . . . . . 128 Solid state UV-Vis spectra of PCDTBT/PC61 BM(1−x) /C60(x) films with various fraction of C60 . . . . . . . . . . . . . . . . . . . . . . 130 (a) Transfer characteristics and (b) Mobility as a function of VGS Vth for various D/A ratios and for PCDTBT/PC61 BM(0.6) /C60(0.4) . 130 Energy levels of PCDTBT/PC61 BM/C60 - OSCs in an inverted architecture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 Electrical parameters of OSCs with various fraction of C60 : (a) Voc , (b) Jsc , (c) FF and (d) PCE. . . . . . . . . . . . . . . . . . . 132 Normalized PCE as a function of the time of annealing treatment at 160o C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 Microscopic images of PCDTBT/PC61 BM(1−x) /C60(x) films thermally treated at 160 o C for 2 hours, 4 hours and 8 hours (magnification 20). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 Microscopic images of PCDTBT/PC61 BM(1−x) /C60(x) films thermally treated at 160 o C for 2 hours, 4 hours and 8 hours (magnification 50). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

xxi

List of Tables 1.1

HOMO LUMO levels of acceptor materials. . . . . . . . . . . . . .

15

2.1 2.2

Boiling points and solubility limits of PC61 BM in various solvents. Effects of processing additives on the PCE of several types of D/A OSCs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

22

4.1 4.2 4.3 4.4 4.5

5.1 6.1 6.2 7.1 7.2

7.3 7.4 7.5

31

HSPs of P3HT and PC61 BM. . . . . . . . . . . . . . . . . . . . . Characteristics of processing additives: HSPs, RED with P3HT and PC61 BM, reported PCEs. . . . . . . . . . . . . . . . . . . . . Characteristics of processing additives: HSPs, REDs with P3HT and PC61 BM and boiling points. . . . . . . . . . . . . . . . . . . . Solubility of P3HT and PC61 BM. . . . . . . . . . . . . . . . . . . Photovoltaic properties of OSCs. For each processing additive, the table shows the photovoltaic properties of OSCs containing the processing additive at a concentration giving the best PCE. .

55

Values of the FWHM of P3HT diffraction peak at 5.5° for different active layer compositions. . . . . . . . . . . . . . . . . . . . . . .

61

Mobility of ODT. Mobility of DPH.

of holes and electrons . . . . . . . . . . . . of holes and electrons . . . . . . . . . . . .

as a function of . . . . . . . . . as a function of . . . . . . . . .

the concentration . . . . . . . . . . . the concentration . . . . . . . . . . .

48 50 51 53

82 84

Initial electrical performances of OSCs with different types of active layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Normalized electrical performance after light irradiation in air: 252 hours of light irradiation for OSCs with processing additives, 90 hours for thermally annealed OSCs. . . . . . . . . . . . . . . . 97 Normalized photovoltaic parameters of OSCs after 393 hours of light irradiation in inert atmosphere. . . . . . . . . . . . . . . . . 99 Decrease in optical density measured at 601 nm. . . . . . . . . . . 102 Normalized photovoltaic performance of C-PYR, DPH and ODTtreated OSCs and thermally annealed OSCs subjected to light irradiation in inert atmosphere for 40 hours. . . . . . . . . . . . . 106

xxii

List of Tables

7.6 7.7 7.8 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9

Force loss in the adhesion measurement test of thermally annealed samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 Force loss in the adhesion measurement test of ODT-treated samples.110 Roughness of active layers with different processing additives. . . 111 HSPs of PDQT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 Photovoltaic parameters of OSCs with various ratios of PDQT/PC61 BM.117 Diffraction peaks and domain sizes of PDQT/PC61 BM with various D/A ratios. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 Electrical performance of PDQT/PC61 BM-OSCs with and without additive. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 HSPs of PCDTBT. . . . . . . . . . . . . . . . . . . . . . . . . . . 125 Properties of PCDTBT/PC61 BM active layers investigated. . . . . 126 Absorption maximum of PCDTBT/PC61 BM films with or without processing additive. . . . . . . . . . . . . . . . . . . . . . . . . . . 128 Average electron mobility measured in a saturation regime at VGS Vth = 4 V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 Normalized photovoltaic parameters of ternary blend OSCs after two hours of thermal treatment at 160 °C. . . . . . . . . . . . . . 134

xxiii

Abbreviations A

Acceptor

AFM

Atomic Force Microscopy

AM 1.5

Air Mass 1.5

ATR

Atenuated Total Reflection

BHJ

Bulk Heterojunction

C60

Buckminster fullerene

CB

Chlorobenzene

C-PYR

1-cyclohexyl-2-pyrrolidinone

D

Donor

DDP

1,4-diketopyrrolo[3,4-c]pyrrole

DEGDE

di(ethylene glycol)-diethyl ether

DPH

Dimethyl Phthalate

DPPBT

Poly(diketopyrrolopyrrole-quaterthiophene

DPPT

Poly(diketopyrrolopyrrole-terthiophene)

ETL

Electron Transport Layer

FF

Fill Factor

FWHM

Full Width at Half Maximum

GIXD

Grazing Incidence X-ray Diffraction

IDS

Drain source current

HOMO

Highest Occupied Molecular Orbital

HSP

Hansen Solubility Parameters

HTL

Hole Transport Layer

ICBA

Indene-C60 bisadduct xxiv

IC70 BA

Indene-C70 bisadduct

ITO

Indium Tin Oxide

J

Current density

J -V

Current density - Voltage

Jsc

Short-Circuit current density

LUMO

Lowest Unoccupied Molecular Orbital

MoO3

Molybdenum oxide

NMP

n-methyl-2-pyrrolidinone

ODCB

O-Dichlorobenzene

ODT

1,8-octanedithiol

OSC

Organic Solar Cell

OTFT

Organic Thin Film Transistor

P3HT

Poly-3-hexylthiophene

PC61 BM

[6,6]-phenyl-C60-butyric acid methyl ester

PCDTBT

Poly[N-9’-heptadecanyl-2,7-carbazole-alt-5,5-(4’,7’-di-2-thienyl-2’,1’,3’-ben-zothiadiazole)]

PCPDTBT

Poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethyl-hexyl)-4H-cyclopenta[2,1-b:3,4-b0]dithiophene-2,6-diyl]]

PCE

Power Conversion Efficiency

PDQT

Diketopyrrolopyrrole (DPP) with β- unsubstituted quaterthiophene (QT)

PDTSTPD

Poly(4,4’-bis(2-ethylhexyl)-dithieno[3,2-b:2’,3’-d]silole)-2,6-diyl-alt-((5-octyl-thieno[3,4-c]pyrrole-4,6-}dione)-1,3-diyl)]

Pedot-PSS

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid)

PM-IRRAS

Polarization Modulated-Infra Red Reflection Absorption Spectroscopy

PTB7

Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b9]dithiophene-2,6-diyl]

PVT

Poly(1-vinyl-1,2,4-triazole)

RED

Relative Energy Difference

Rsh

Shunt Resistance

Rs

Series Resistance

SCLC

Space Charge Limited Current xxv

TRIB

Tributyl o-acetylcitrate

UV-Vis

UltraViolet-Visible

VDS

Drain source voltage

VGS

Gate source voltage

Voc

Open Circuit Voltage

Vth

Threshold Voltage

XRD

X-Ray Diffraction

ZnO

Zinc Oxide

xxvi

Symbols δ

Solubility parameter [MPa1/2 ]

eV

Electron Volt

µ

Mobility [cm2 .V−1 .s−1 ]



Resistance [Ohm]

xxvii

Chapter 1

Introduction to organic solar cells

1

Introduction to organic solar cells

1.1

General introduction

The industrial revolution represented a serious transition in humanity’s standard of life leading to significant advances that contributed to higher life expectancy, quality of life and comfort. These developments, along with a growing population, caused exponential growth in energy demand over the past years. To date, energy demand has mostly been met by burning fossil fuels (oil, gas and coal). However, such processes are not environmentally sustainable due to the emission of green house gases in Earth’s atmosphere, contributing to global warming. Additionally, as fossil fuel is a non-infinite source of energy, the resource depletion causes serious societal and political issues. Alternatives must be developed to satisfy the energy demand in a sustainable way. In this context, solar energy offers many advantages as a renewable source of energy. One of them is the large amount of energy provided by solar irradiation on Earth: estimations - considering realistic irradiated surface and conversion yield - showed that solar energy could provide up to two times the world’s energy demand [54]. Additionally, the worldwide availability of solar energy could benefit remote regions of the world by means of decentralized production of energy. Solar radiation is converted into electricity by means of photovoltaic cells, also called solar cells. They have been fabricated from a wide range of materials as shown in the National Renewable Energy Laboratory chart (Figure 1.1). To date, the most prevalent types of photovoltaic panels use crystalline silicon in the monocrystalline or polycrystalline form. At the laboratory scale, conversion efficiencies reach 25% [64], while in commercial products, the efficiencies tend to be around 20% [103]. Crystalline silicon solar cells dominate in terms of efficiency, but suffer from high fabrication costs and restrictive mechanical and physical properties, such as weight and fragility. Therefore, they have not been able to fully compete with fossil fuels. In the pursuit of further lowering module cost and encouraging wide-spread application, other technologies were developed. Thin film technologies (indicated by data in green in Figure 1.1) are a good alternative as they require less material and lower fabrication costs. Organic photovoltaics (depicted in red in Figure 1.1) are also an interesting and exciting route for the prospect of extremely low cost solar panels with additional features such as low weight and flexibility. 2

Introduction to organic solar cells

Figure 1.1: Photovoltaic device efficiency within different families of semiconductors. Reproduced from reference [1].

The history of organic photovoltaics started with the discovery of conductive properties in polymers by Heeger, MacDiarmid and Shirakawa in 1977 for which they were awarded the Nobel Prize in Chemistry in 2000 [77]. Polymers were traditionally seen as insulating materials, however, the presence of alternating π-conjugated bonds can confer semiconductive properties to polymers. Research and development in the field of organic solar cells (OSCs) are relatively new compared with traditional electronics. The first double-layer type of organic photovoltaic device was demonstrated in 1986 by Tang et al. with a power conversion efficiency of 0.95% [204]. In 1995, solution processed organic photovoltaics were fabricated for the first time from a blend of polymer and oligomer [235]. Until the early 2000s, the maximum power conversion efficiencies of organic photovoltaics remained around 3% [48, 179, 191]. For a long time, these low efficiencies were seen as a significant barrier to commercialization. However, much research was dedicated to improve the fundamental and technical knowledge in the field, which led to significant improvements in performance. Today, organic photovoltaics demonstrate their potential in becoming a market reality by exhibiting efficiencies greater than 10%. Heliatek, a spin-off from a University of Dresden 3

Introduction to organic solar cells

laboratory in Germany is currently holding the power conversion efficiency record of 12% with a multi-junction OSC. Efficiencies approaching 10% were obtained by several groups with single-junction devices [71, 76, 156]. From a technological point of view, the physico-chemical properties of organic semiconductors open up a new route in the development of photovoltaic devices. One of the key properties is the ability to dissolve organic semiconductors in solvents, which enables OSCs to be fabricated using solution processes similar to those used in the printing industry [9, 20, 26, 33, 58, 60, 106, 107, 154]. Such processes mean easier fabrication of large area and low cost photovoltaics and consequently, a number of innovative devices can be considered, including flexible, transparent, and light weight devices as depicted in Figure 1.2 [34, 87, 194]. In addition, the fabrication of OSCs has a lower energy payback time in the long term and a smaller environmental impact than other technologies [54]. Overall, they represent a promising technology for low cost, accessible and sustainable energy.

(a)

(b)

Figure 1.2: Photographs of (a) an OSC printed on paper, reproduced from reference [87] with permission of Wiley and (b) a transparent OSC, reprinted with permission from [34]. Copyright (2012) American Chemical Society

To become a competitive technology, OSCs need to improve in three fronts: efficiency, lifetime and fabrication cost. Research, with the goal of improving in these areas, has been focused on the synthesis of organic semiconductors, their processing and the development of smart device architectures. All three topics contribute to the overall performance of the final device (Figure 1.3). The past decade was marked by a tremendous increase in types of organic semiconductors [41, 109], which indubitably contributed to the current success of organic photovoltaics. However, the structural properties of materials alone are not sufficient to produce high efficiency devices. The arrangement, e.g. the morphology, of the materials is of crucial importance in defining their performance in 4

Introduction to organic solar cells

Building blocks Material synthesis

Efficiency Material Processing Device Fabrication

Device Value Stability

Cost

Figure 1.3: Building blocks for device fabrication and the three major axes of performance.

devices. High efficiency OSCs require a thorough control of the semiconductor morphology to best assess their potential in a device configuration. In this context, this thesis addresses the interplay between processing method and semiconductor morphology and subsequently photovoltaic performance. The effects of processing are assessed by means of semiconductor formulation and introduction of processing additive in the formulation. The first chapter of this thesis provides an introduction to fundamental principles of organic semiconductors and to the field of organic photovoltaics. Chapter 2 presents a detailed literature review on the current state of knowledge on formulation strategies to control active layer morphology. Emphasis is placed on the system based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61 BM) as it is the main model system in this thesis. The objectives of this thesis are described at the end of Chapter 2. Chapter 3 details the experimental and analytical methods used throughout the thesis. Chapter 4 reports a novel method for identifying efficient processing additives to be introduced in the formulation of organic semiconductors. Details about the selection method and results confirming the model are presented. Chapter 5 focuses on the mechanistic effects of processing additives on the morphology and efficiency. Results of structural, optical and electrical characterizations are reported and a mechanism describing the effects of processing additives 5

Introduction to organic solar cells

on the morphology is proposed. Chapter 6 presents results on the influence of device architecture on the performance of additive-treated OSCs. The effects of processing conditions on device stability is addressed in Chapter 7. Results of stability tests performed on OSCs processed under different conditions are reported. Also, investigation on the origin of device degradation is presented. Chapter 8 reports on the investigation of formulation strategies to increase the efficiency of two polymers, other than the model polymer P3HT discussed in the previous chapters. Chapter 9 summarizes the results and provides suggestions for future work.

6

Introduction to organic solar cells

1.2

Functioning of solar cells

Before introducing the concept of OSCs, this section discusses the electrical functioning of solar cells and introduces the parameters that characterize their performance. The simplified structure of a solar cell consists of two electrodes separated by a photo-active layer which contains a p-n junction. A solar cell without any irradiance (dark characteristic) behaves as a diode as shown in Figure 1.4. When a photon hits the active layer, a bond state of an electron and a vacant site of an electron called a hole is formed. The separation of the bound state, also referred to as exciton, generates charge carriers that can travel across the active layer to be collected by the electrodes. The extraction of these photo-generated charge carriers by an external load constitutes the photo-current (Jph ). Under illumination, the diode characteristic shifts by the amount of the photo-current. 6 D A R K IL L U M IN A T IO N

Current density [mA/cm²]

4 2 0

V -2

V

-4

J

J

P H

-6

m

V

O C

O C

m

-8

-1 0

J

-1 2 -0 .4

-0 .2

S C

0 .0

0 .2

0 .4

0 .6

V o lta g e [V ]

Figure 1.4: Example of a typical solar cell characteristic under light illumination and under dark condition.

The total current density through the solar cell can be analyzed using an equivalent circuit model depicted in Figure 1.5. The equivalent circuit model consists of four parts: a photo-current source, a diode, a series resistor and a shunt resistor. The diode represents the holeelectron recombination current and the photo-current is the amount of extracted photo-generated charges. The series resistor represents the solar cell internal 7

Introduction to organic solar cells Diode

Series Resistor

Shunt Resistor

Jph

V

Figure 1.5: Equivalent circuit model for a solar cell.

resistance and the shunt resistor models any leakage current through the device. The total current density J flowing through the circuit can be described by the following relationship: J = JD (V ) +

V − JRserie − Jph Rshunt

(1.1)

Where Rs is the series resistance, Rsh the shunt resistance and JD the current characteristic for a diode. The electrical parameters that define the efficiency of a solar cell are extracted from the current density-voltage (J-V ) curves measured under light illumination. Devices are characterized by the short-circuit current (Jsc ), the open-circuit voltage (Voc ) and the fill factor (FF). The Voc and the Jsc are indicated in the typical J-V curve depicted in Figure 1.4. The Jsc is defined as the current at which the applied voltage is equal to 0 V. This parameter represents the number of charge carriers that are photo-generated and collected at the electrodes at short-circuit condition. The Voc is defined as the voltage at which the current density is 0 mA.cm−2 . The FF defines the shape of the J-V curve and is defined as: FF =

Jm .Vm Jsc .Voc

(1.2)

where Jm and Vm are respectively the current density and the voltage at the point of the maximum output power, as represented in Figure 1.4. Finally, the power conversion efficiency (PCE) of a solar cell can be calculated as follows: P CE =

Voc .Jsc .F F Pin

8

(1.3)

Introduction to organic solar cells

where Pin is the input power density. Solar cells are typically characterized under 100 mW.cm−2 light of the Air Mass 1.5 Global (AM 1.5 G) solar spectrum. The AM 1.5 G spectrum is presented in Figure 1.6, it corresponds to the solar spectrum through atmosphere, 48.2° from

A M

1 .5

1 .5 G

S p e c tr a l I r r a d ia n c e [ W .m

-2

.n m

-1

]

zenith.

1 .0

0 .5

0 .0

5 0 0

1 0 0 0

1 5 0 0

2 0 0 0

2 5 0 0

3 0 0 0

3 5 0 0

4 0 0 0

W a v e le n g th [n m ] Figure 1.6: AirMass 1.5 Global solar spectrum.

1.3 1.3.1

Organic semiconducting materials Electronic properties of organic semiconductors

Organic materials are primarily carbon-based. The fundamental property of organic semiconductors is the presence of a conjugated π-electron system within a carbon chain. In such a carbon chain, three out of the four valence electrons of each carbon atom occupy sp2 hybridized orbitals and are involved in covalent σ-bonds. The remaining valence electron occupies a pz orbital and can form a π-bond with a pz electron of a neighboring carbon. As a result, the carbon chain exhibits an alternation of single (σ) and double (σ and π) bonds which constitutes

9

Introduction to organic solar cells

the conjugated π-electron system. The simplest example of a π-conjugated system is that of polyacetylene, illustrated in Figure 1.7, which consists of repeating ethylene units.

Figure 1.7: Chemical structures of: (a) ethylene, (c) polyacetylene and the overlapping of orbitals in (b) ethylene and (d) polyacetylene.

Within a π-conjugated system, the overlap of the wave functions of the pz orbitals of the carbon atom (and also of other atoms such as nitrogen, oxygen, sulfur) results in the formation of the π-band (forming the highest occupied molecular orbital (HOMO)) and the π*-band (forming the lowest unoccupied molecular orbital (LUMO)). Across the conjugated system, the π-electrons are delocalized. The difference between the HOMO and the LUMO levels defines the bandgap of the semiconductor. The gap narrows down as the conjugation length of the polymer chain increases. In molecular solids, the degree of intermolecular ordering has a large impact on the energetic landscape in these materials and thus affects the band gap. Overall, increased conjugation length and intermolecular ordering cause a greater degree of electron delocalization while short conjugation length and intermolecular disorder localize electrons. The specific electronic structure of organic semiconductors leads to major differences between the properties of organic and conventional semiconductors. An understanding of these differences, described in brief in the following sections, is essential to provide insights into understanding the design requirements for OSC active layers.

1.3.2

Charge carrier generation in organic semiconductors

Free charge carriers in semiconductors are generated by light, chemical doping, or injection of charge from an electrode. The generation of charge carriers by 10

Introduction to organic solar cells

light occurs when a photon with an energy equal to or higher than the band gap hits the semiconductor, resulting in the photo-generation of an exciton. To generate free charge carriers, the bound electron-hole pair in the exciton must be separated. In a traditional inorganic semiconductor, the large dielectric constant (around 10 [55]) reduces the Coulomb interaction that binds the hole and the electron. This results in a large-sized type of exciton called a Wannier-Mott exciton characterized by an exciton binding energy in the range of ∼ 0.01 eV. This type of exciton can be dissociated by thermal energy at room temperature as the binding energy is lower than the thermal energy (0.025 eV). In contrast, in organic semiconductors, the dielectric constant is much smaller (around 3 [55]) causing a strong Coulombic interaction between the hole and the electron. This results in a small-sized exciton with a high binding energy (from hundreds of meV to 1.5 eV [10, 82, 93]) referred to as a Frenkel exciton. This high binding energy prevents the exciton from being dissociated by thermal energy at ambient temperature. One alternative for exciton dissociation is to introduce a second semiconductor with a different electron affinity. In organic semiconductors, the charge separation is generally a two-step process, where excitons are first separated into less strongly bound electron-hole pairs which can subsequently be dissociated.

1.3.3

Charge transport in organic semiconductors

In crystalline inorganic semiconductors with a 3D crystal lattice, atoms are held together by strong covalent bonds. The strong interatomic interaction leads to the formation of a conduction band and a valence band in which the free charge carriers are highly delocalized and can travel freely throughout the bands [196]. In contrast, in disordered organic semiconductors, such as π-conjugated polymers, charge carriers are localized on individual molecules because of poor intermolecular coupling. The charge transport is generally described by the variablerange hopping model [98, 197] which describes systems with mobilities around or below 10-2 cm2 .V-1 .s-1 [98]. The mobility measures the ease with which the charge carriers move in the system and corresponds to the drift velocity of the charge carrier in an electrical field per unit of electrical field as described in the following equation [21]: µ=

eD Kb T

11

(1.4)

Introduction to organic solar cells

Where D is the diffusion coefficient, e the charge, Kb the Boltzman constant and T the temperature. In π-conjugated polymers, the hopping transport is highly influenced by the conjugation length and the degree of intermolecular order [21]. The mobility of charge carriers in organic semiconductors thus appears to be dependent not only on the chemical structure of the semiconductors but also, and just as importantly, on their intermolecular arrangements. Generally, for semiconducting polymers processed from solution, mobilities in the range of 10-3 - 10-6 cm2 .V-1 .s-1 are obtained [98]. A control over molecular organization can improve the mobility [200]. In the case of highly-ordered organic molecular crystals, charge carriers are sufficiently delocalized for band transport to occur [154].

1.4 1.4.1

Organic solar cells Active layer architectures

As previously mentioned, thermal energy is not sufficient to dissociate Frenkel type excitons photo-generated in organic semiconductors. Instead, a junction must be created at which it is energetically favourable for an exciton to dissociate. The first type of OSCs consisted of single layer of semiconductor that formed a rectifying junction with one of the electrodes [12, 28]. In this architecture, because of the high exciton binding energy and the low exciton diffusion length (1 - 10 nm) in organic semiconductors [72, 141, 199], the charge generation only occurs near the interface between the electrode and the active layer. This resulted in poor photo-generation of charge carriers and high series resistance as one of the charge carriers has to travel across the entire active layer to be collected. These issues were later addressed using a heterojunction between an electron donating material (D) and an electron accepting material (A) to form a so-called bi-layer. In a D/A couple, the A is the material with the largest electron affinity and the D the material with the lowest ionization potential. The energy offset at the D/A interface helps to dissociate excitons. The first heterojunction device, developed in 1986, consisted of a bi-layer of an A (tetracarboxylic derivative) and a D (copper phthalocyanine) material. These bi-layer OSCs demonstrated efficiencies 12

Introduction to organic solar cells

approaching one percent (0.95%) [204]. However, the photo-generation of charges was still limited to occur at one single interface and excitons formed further away from the D/A interface do not dissociate into free charge carriers, therefore recombine, and thus do not contribute to photo-current. A significant improvement with respect to bi-layer devices is the Bulk Heterojunction (BHJ) concept. A BHJ type of active layer consists of an intimate mixture of a D and an A. Such mixtures result in many D/A interfaces throughout the entire active layer and thereby provide a compromise between the amount of interface for exciton separation and D/A pathways for hole/electron transport. Figure 1.8 depicts the schematic of a bi-layer and a BHJ type of active layer. In 1992, Sariciftci et al. revealed a fast photoinduced charge transfer between

h+ e-

P type N type

h+

e-

h+ e-

(a)

(b)

Figure 1.8: Simplified schematic of (a) a bi-layer active layer and (b) a BHJ-active layer.

poly(2-methoxy-5-(2-ethyl- hexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and buckminsterfullerene (C60 ) [183]. In 1995, solution-processed BHJ-OSCs were fabricated using the property of fast charge transfer between MEH-PPV and PC61 BM, a soluble derivative of C60 [235]. A blend of MEH-PPV and PC61 BM was dissolved in xylene and spin-cast to give a thin film consisting of an interpenetrated network between the two materials: a BHJ. The photovoltaic performance revealed that blending PC61 BM with MEH–PPV polymer caused the PCE to increase by two orders of magnitude compared with an OSC composed of MEH-PPV only. This BHJ concept is now widely adopted and has been the subject of significant amount of research aimed at increasing the efficiency of solution-processed OSCs.

13

Introduction to organic solar cells

1.4.2

Formation of BHJ-OSCs

In polymeric BHJs, the D semiconductor is a polymer and the A semiconductor a small molecule. Several examples of A small molecules and D polymers are shown in Figure 1.9.

C60

PC60BM

P3HT

PC70BM

PCDTBT

ICBA

PCPDTBT

Figure 1.9: Top row: examples of A small molecules and bottom row: example of D polymers.

C60 and its derivatives are the most widely used A small molecules. C60 is strongly electronegative and can accept up to six electrons. Additionally, it exhibits a small reorganization energy from the ground state to the radical anion upon electron transfer which enables fast photoinduced electron transfer and slows down charge recombination compared to other types of A such as quinone derivatives [67, 88]. As the solubility of C60 is low in common organic solvents [189], soluble derivatives such as PC61 BM are preferred for solution processed applications. Their HOMO LUMO energy levels are shown in Table 1.1. PC61 BM and other fullerene derivatives such as [6,6]-phenyl-C71-butyric acid methyl ester (PC71 BM) and indene-C60 bisadduct (ICBA) are the best performing A small molecules to date. Alternatives have been investigated but have not yet been able to compete with them [174, 177]. On the other hand, D polymers have been the subject of much research resulting in a large panel of π-conjugated polymers. Examples are shown in Figure 1.9.

14

Introduction to organic solar cells Table 1.1: HOMO LUMO levels of acceptor materials. Materials PC61 BM C60

Energy Levels [eV] HOMO LUMO 6.1 4.3 5.8 3.9

References [100, 126] [153, 173]

To form a BHJ, the D polymer and the A small molecule are generally mixed together in a solvent and the resulting blend is coated on a substrate. The drying process involves various phenomena including self-organization of the materials and phase separation between D and A. The resulting phase separation and degree of order in the BHJ govern the conversion from light absorption to charge collection and are therefore crucial to control. It is noteworthy to point here that few studies in the literature focused on the formation of BHJs with fixed morphology by the mean of various methods such as the use of D or A nanoparticles [61] or nanowires.[228, 229] In these approaches, the domain sizes in the BHJ are mainly determined by the size of the initial objects and are less dependent on the conditions of the drying process. This in turn enables a large degree of control in the BHJ morphology under the condition that the sizes of the initial objects are fully controlled.

1.4.3

Functioning of BHJ-OSCs

Within the interpenetrated network formed by the D and A, the conversion from light to electrical current occurs according to the following steps: (i) Absorption of photon (ii) Formation of a Frenkel exciton (iii) Diffusion of the Frenkel exciton to a D/A interface (iv) Separation of the exciton via charge transfer between D and A resulting in a charge transfer state (v) Dissociation of the charge transfer state into free charge carriers (vi) Transport of free charge carriers: the electrons through the A and the holes through the D (vii) Collection of the charge carriers at the electrodes. 15

Introduction to organic solar cells

When an OSC is subject to light illumination, photons with appropriate energy are absorbed and cause the photo-generation of Frenkel excitons in the active layer. Because of its high exciton binding energy, the Frenkel exciton can only dissociate at the interface between D and A. This means that only excitons photo-generated within the exciton diffusion length from a D/A interface can contribute to the generation of free charge carriers. Excitons that are formed within a larger distance from this interface undergo geminate recombination and are lost. At the interface, charge transfer from the D to the A is possible if the energy offset at the interface is sufficiently large. Charge transfer occurs if the LUMO level of the A lies between the HOMO-LUMO levels of the D as shown in Figure 1.10. The energy offset between the LUMO of the D and the LUMO of the A has to be greater than 0.3 eV for charge transfer to occur [190]. At the interface, the exciton dissociates to form the charge transfer state corresponding to the e− /h+ pair state in which the electron is located on a neighbor molecule of the molecule on which the hole is located. The dissociation of the charge transfer state generates a free electron and a free hole which are then transported through A or D pathways to their respective electrode under the influence of an internal electric field. In practice, a hole transport layer (HTL) and/or an electron transport layer (ETL) are inserted between the active layer and the cathode and anode respectively to facilitate the process of charge collection and to prevent additional charge recombination at the electrodes. The general energy band diagram of BHJ-OSCs that describes the above mechanisms is depicted in Figure 1.10. LUMO LUMO

Charge transfer Light

eh+

HOMO HOMO

Cathode HTL

D

A

Anode

Figure 1.10: Energy diagram in BHJ-OSCs. The figure depicts a situation where the light is absorbed by the D. The photogeneration of the exciton is followed by an electron charge transfer from the LUMO of the D to the LUMO of the A.

16

Introduction to organic solar cells

The microstructure of the BHJ, e.g. the morphology, affects the efficiency of the different mechanisms described above [24]. The morphology of a BHJ is defined by (1) the phase separation between A and D and (2) the molecular organization. For high performing OSCs, both charge generation and charge collection need to be considered. These two processes are in competition: charge generation requires small D/A domains to provide a large amount of D/A interface, while charge collection requires large pathways that subsequently limit the amount of D/A interfaces. A compromise is obtained when D and A domain sizes are in the range of the exciton diffusion length. The exciton diffusion length, which is in the range of 1 to 10 nm [72, 141, 199] implies that D and A domain sizes of about 10 nm offer a decent compromise. [68]. In addition to phase separation, the molecular organization within D or A domains is important. In conjugated polymers, the interlayer spacing between polymer chains, the degree of ordering and the orientation of the chains have an influence on charge carrier mobility. In OSCs, charge carrier mobility is closely related to recombination processes [158, 165]. Studies revealed that charge carrier mobility value along with a balance between electron and hole mobility contribute in improving the performance of OSCs [3, 38, 101, 113, 158].

1.4.4

Factors influencing BHJ-OSC efficiency

The previous sections describe the mechanisms related to the functioning of BHJOSC and the importance of the BHJ morphology. This section addresses the parameters characteristic of OSCs. - Jsc The Jsc is given by the amount of charge carriers that are collected at the electrodes. This parameter is therefore depends on the amount of photons absorbed and to the loss mechanisms that prevent charge collection. The morphology of the BHJ has a great impact on the amount of photo-generated charges, due to the amount of D/A interface, and on the collection of charges by the presence of D or A pathways. Recent findings have highlighted the important role of the intermixed phase in the BHJ in exciton quenching and charge generation processes [15, 40, 193, 205, 223]. In this intermixed phase, the D and the A are 17

Introduction to organic solar cells

molecularly mixed and form amorphous domains. Studies have shown that it is crucial for these intermixed phases to coexist with the relatively pure D and A domains that stabilize charge separation and enable transport to the electrodes. - Voc Several works have ascribed the dependence of the Voc to the spectral position of the charge transfer state [111, 133, 209]. The energy of the charge transfer state originates primarily from the energy difference between the HOMO of the D and the LUMO of the A (∆EDA ). ∆EDA determines the theoretical maximum value of Voc [22, 185, 224]. Experimentally, the Voc is observed to be lower than the value set by ∆EDA because of several loss mechanisms at the contacts or in the active layer [209, 232]. In their early work, Scharber et al [185]. investigated the relationship between the energy levels of the D/A blends and the Voc in 26 BHJ-OSCs. They found that the Voc of the OSCs could be estimated by: 1 V oc = (|E Donor HOM O| − |E P CBM LU M O|) − 0.3 e

(1.5)

where e is the elementary charge and the value of 0.3 an empirical factor accounting for losses in OSCs. Regarding the loss at the contacts, the properties of the HTL (energy levels and conductivity) were shown to have an influence on the built-in voltage in the device and subsequently on the Voc [207]. In the case of ohmic contacts, the loss in Voc at the contact was attributed to diffusion of charge carriers into the active layer at the interface with the metal electrodes resulting in a reduction of the voltage at which the flat band conditions are reached [210]. Regarding the active layer, several loss mechanisms that affect the charge transfer band position were identified. The charge transfer band was found to be dependent on the electrostatic environment, which can be altered by the concentration of PC61 BM nanocrystals because of their high dielectric permittivity [133, 175]. Also, the level of polymer aggregation affects the position of the charge transfer state because of its dependence on intermolecular interactions [172]. Strong intermolecular interactions were shown to result in low Voc whereas low intermolecular interactions obtained in the case of amorphous polymers, for example - lead to higher Voc .

18

Introduction to organic solar cells

- FF The origins of low FF generally arise from space-charge build up. The spacecharge regions can arise in the vicinity of the active layer and the contacts, in the case of poor charge extraction at the electrodes. In this context, the nature of the charge transport layers between the active layer and the electrodes have the important role of preventing charge accumulation [163]. Efficient contacts and interlayers facilitate the extraction of charges and reduce bimolecular recombination in the active layer. Space-charge build up can also be observed in the case of increasing light intensity [111, 227], or unbalanced charge carrier mobility [3, 109]. The value of the FF is generally found to be dependent on the values of Rs and the Rsh measured in OSCs [97, 172]. This chapter provided an overview of the mechanisms behind OSCs and more specifically of BHJ-OSCs. The next chapter focuses specifically on the formation of that morphology and strategies to control and optimize it.

19

Chapter 2

Formulation strategies for controlling BHJ morphology

20

Formulation strategies for controlling BHJ morphology

This chapter reviews the benefits and the limitations of three main strategies that can control the morphology of BHJ-active layers: the solvent choice, the use of post-processing steps and the use of processing additives. The objectives of the thesis are presented at the end of the chapter.

2.1

Role of solvent

The BHJ morphology is strongly dependent on thermodynamic and kinetic aspects involved during spin-casting and drying of the material formulation to form the thin film [144, 233]. The thermodynamics of the system is dictated by the intrinsic properties of the D and A materials (their tendency to crystallize, their interactions with each other and their miscibility) and by the properties of the solvent. The kinetics of drying primarily depend on the boiling point and the vapor pressure of the solvent. The effects of the solubility properties and the boiling point of solvents are reviewed below.

- Solubility properties D and A solubility properties in solvent were found to affect the aggregation of the fullerene derivatives in the D/A films. Table 2.1 displays the solubility limits of PC61 BM in some conventional solvents including chloroform (CF), chlorobenzene (CB), o-dichlorobenzene (ODCB) and 1,2,4-trichlorobenzene (TCB). The study by Brabec et al. on a blend of MDMO-PPV and PC61 BM was among the first to bring some insights into the relationship between solvent quality and the morphology of a BHJ [191]. In the D/A system based on MDMO-PPV/PC61 BM, the size of PC61 BM aggregates were found to be dependent on the solubility limit of PC61 BM in the solvent [57, 84, 85]. When poor solvents are used (such as pyridine and toluene), the BHJ exhibits large size aggregates of PC61 BM in the order of hundreds of nanometers. In the films spin-cast from good solvents, such as CB or ODCB, the large features are absent. Figure 2.1 shows atomic force microscopy (AFM) images that clearly depicts the dependence of PC61 BM aggregate sizes on solvent quality. This morphology is attributed to the fact that in good solvents, PC61 BM molecules remain finely dispersed and no over-sized

21

Formulation strategies for controlling BHJ morphology

aggregates are formed. Smooth films of MDMO-PPV/PC61 BM exhibited the highest PCEs, in the order of 2.5%. Table 2.1: Boiling points and solubility limits of PC61 BM in various solvents. JOURNAL OF POLYMER SCIENCE

FULL PAPER

WWW.POLYMERPHYSICS.ORG

Solvents Boiling point Solubility limit TABLE 2 Summarized Hansen Solubility Parameters for Various Organic Semiconductors dD (MPa½)

CF

PC61BM PC71BM

CB

19.33 6 0.05

ODCB

19.64 6 0.32

F8-NODIPS DPP(PhTT)2

18.48 6 0.28

MDMO-PPV

19.06

MEH-PPV

19.06

TCB

P3HT PFO

61 °C

20.16 6 0.28

DPP(TBFu)2

Polymers

19.89 6 0.34

18.55

Xylene

5.68 6 1.03

Carbon Disulfide

−1 6 0.92 25 - 26 3.64 mg.mL

6.6

½

4.49 6 0.57

−1 6 0.48 132 °C 4.78 6 0.5025 - 59.56.26mg.mL

183 °C

3.24 6 0.91

8.1

6.12 6 0.65



5.28

5.5

5.62

214 °C 5.38

−1 81.4 mg.mL 5.44

2.88

0.15

0.59 [32, 51, 57, 181]

5.1

3.54 6 0.56

42 – 107 mg.mL−1

RED (PC61BM)

0.00 181] [32, 51, 57,

7.0

2.62 6 0.56

0.52



[57, 162] 0.42

6.0

[162] 0.41 1.08

3.19

3.6

112 °C 2.8

9 - 15.64.51 mg.mL−1

4.1

138 C

PC BM −1 through other 5 diffusion - 22.1ofmg.mL [32, conjugated 51, 57, polymers; 181]

163 C

however, our data suggest that the diffusivity of PC61BM may be affected by the−1 solvent used to process the film.24 Longer annealing time would lead to larger aggregates.45

and < 50 nm for pyridine). In stark contrast°to the fused nanoclusters, porous networks of MDMO-PPV polymers (average pore size in these films is calculated to be 190 6 32 ° nm) and micron-sized aggregates are observed in the annealed films deposited from CS2. This is consistent with the film optical data and suggests that the PC61 ° BM domains in the underlying film are quite mobile and can quickly diffuse out of the internal structure to form larger aggregates. The diffusion of PC61BM is consistent with reports on the

Mesitylene

R0

H

5.37 6 0.80

18.56

Toluene

d (MPa ) of PC 61 BM

dP (MPa½)

Small molecules

References

46 C

0.98 181] [32, 51, 57,

61

47 - 48.1 mg.mL

[32, 51]

The morphologies of−1 MEH-PPV:PC61BM (Supporting Information Fig. S5) are in striking contrast to those of MDMO-PPV, even though there is little structural difference between the two polymers (3,7 dimethyloctyl vs. 2-ethylhexyl solubilizing groups), HSP values or the optical properties of the films.

207 mg.mL

[57]

FIGURE 2 AFM topography images (2 lm  2 lm) of as-cast and annealed films of MDMO-PPV:PC61BM spin-casted from chlorobenzene (a, b), CS (c, d), CHCl (e, f), pyridine (g, h), trichloroethylene (i, j), toluene (k, l), and 1-methylpyrrole (m, n). Figure 2.1: AFM topography images (2 µm x 2 µm) of as-cast and annealed films of MDMO-PPV-PC61 BM spin-cast from (a, b) chlorobenzene (CB), (c, d) carbon disulfide, (e, f) chloroform (CF), (g, h) pyridine, (i, j) trichloroethylene, (k, l) toluene and (m, n) 1-methylpyrrole. Reproduced from reference [57] with permission of Wiley. 2

3

WWW.MATERIALSVIEWS.COM

JOURNAL OF POLYMER SCIENCE PART B: POLYMER PHYSICS 2012, 50, 1405–1413

1409

Similar results were reported with blends of PCDTBT/PC61 BM [16, 166, 192] and P3HT/PC61 BM [181, 206]. In a BHJ of P3HT/PC61 BM, it is well known that thermal annealing causes PC61 BM to crystallize and to form microcrystals [31, 148]. Several studies revealed that thermally annealed BHJs presented the largest 22

Formulation strategies for controlling BHJ morphology

PC61 BM microcrystals when spin-cast from poor solvents of PC61 BM [181, 206]. Clearly, when PC61 BM is blended with P3HT or MDMO-PPV, the crystallization of PC61 BM molecules is governed by its solubility limit. When comparing P3HT and MDMO-PPV, it is worthy to note that the crystallization of PC61 BM seems to depend more on solvent quality when MDMO-PPV is the D. This divergent behavior was attributed to the different strength of the interactions between the polymer and the fullerene components [187]. Overall, the aggregation of PC61 BM is shown to be affected by the D-A interactions and to be dependent on the type of solvent used. The effect of the solvent quality on the polymeric component is another important aspect to consider. A common method to investigate the effects of solvent quality on the behavior of P3HT chains is to add a poor solvent for the polymer (e.g., acetone or hexane) to the host solvent in order to reduce the solubility of the polymer in the solvent system. Keum et al. studied the structural evolution, in solution, of P3HT chains as a function of the solvent quality, using UV-Vis absorption spectroscopy and small-angle neutron scattering [95]. They showed that the introduction of a poor solvent caused the P3HT chains to aggregate and to grow into nanorods in order to reduce the unfavorable solvent-polymer interactions. As a consequence of the preformed aggregates in solution, the crystallinity of P3HT in the dry film greatly improved [186]. Chang et al. used grazing incidence x-ray diffraction (GIXD) to show that the introduction of acetone in the solvent system increased the crystallinity of P3HT (Figure 2.2) causing as much as a 4-fold increase in mobility [32]. Overall, poor solvent quality affects the behavior of the polymer by introducing a driving force for the polymer to aggregate in order to reach a thermodynamically favorable state.

23

Formulation strategies for controlling BHJ morphology

(a) profiles of P3HT films spin-cast from P3HT/CF (b) Figure 2.2: (a) GIXD solutions containing a range of added acetone; (b) 2θ angle (left axis) of (100) peak and corresponding layer spacing (right axis) as a function of the additional acetone volume ratio. Reprinted with permission from [32]. Copyright (2013) American Chemical Society.

- Solvent’s boiling point The organization of the material and the transition from the liquid to the solid state can be affected by the drying kinetics. Using highly volatile solvents, the kinetics of evaporation can be much higher than the kinetics of crystallization. In such cases, the resulting morphology is far from that observed under equilibrium conditions [157]. The effects of the drying kinetics on the amount of polymer chain aggregation have been widely investigated using various deposition techniques [233] and solvents with different boiling points. When P3HT/PC61 BM films were spin-cast from different solvents, it was generally observed that a high boiling point solvent leads to better efficiency [52]. Ruderer et al. studied the morphology of P3HT/PC61 BM films spin-cast from four different solvents (CF, CB, toluene and xylene) [181]. Using grazing incidence wide angle x-ray scattering, they showed that the crystallite sizes of P3HT increased with increasing boiling point of the solvent. Similarly, Verploegen et al. showed that films spincast from CB gave larger crystal sizes than films spin-cast from CF [211]. This was attributed to the fact that a high boiling point solvent resulted in slow drying that provided time for the self-assembly of polymer chains during solvent evaporation. These results indicate that spin-casting from high boiling point solvents allows the P3HT component to arrange in a lower free-energy state compared with thin films spin-cast from a low boiling point solvent. This phenomenon is also observed with several other D polymers such as PCPDTBT and 24

Formulation strategies for controlling BHJ morphology

PTB7 [59, 69]. The examples above show that the choice of solvents has a large impact on the morphology of the BHJ. During solvent evaporation, the drying kinetics and thermodynamics are in competition. For P3HT/PC61 BM blends, high boiling point solvents are preferred to provide time for the polymer chains to self-assemble. An appropriate choice of solvent can, therefore, optimize the morphology of a BHJ. However, there is a only a limited number of solvents that can solubilize both D and A components. In the best solvents, as cast P3HT/PC61 BM-OSCs still exhibit low PCEs generally reported to be below 1.5% [29, 230]. The poor performance is due to fast solvent evaporation caused by the use of spin-coating (or any other deposition technique). The fast solvent evaporation prevents the materials from aggregating and forming proper phase separated domains. As a result, P3HT and PC61 BM form an intimately mixed network lacking of well defined domains. This, in turn, primarly limits charge collection and results in low Jsc and FF. The application of post-processing steps were demonstrated to enable further optimization of the BHJ morphology.

2.2

Effects of post-processing steps

Thermal treatment - also called thermal annealing - on P3HT/PC61 BM-films was shown to significantly increase the degree of crystallinity of P3HT. Figure 2.3 shows the effects of thermal annealing on P3HT/PC61 BM films observed by UVVis absorption spectroscopy and by XRD measurements. UV-Vis absorption spectroscopy represents a useful technique to detect the degree of crystallinity of P3HT because increased crystallinity induces a red-shift in the absorption peak and the appearance of two vibronic peaks around 555 nm and 605 nm [237]. In the XRD pattern in Figure 2.3b, the diffraction peak at 5.5° corresponds to the edge-on orientation of the polymer chains [238]. The appearance of the diffraction peak is a signature of increased crystallinity. The increased degree of P3HT crystallinity is attributed to the fact that the polymer becomes mobile upon annealing and can self-assemble into crystalline domains. This leads to increased ordering between polymer chains. In parallel to the increased ordering of polymer chains, PC61 BM molecules diffuse out of the 25

Formulation strategies for controlling BHJ morphology

(a)

(b)

Figure 2.3: Effects of thermal annealing on the crystallinity of P3HT depicted by (a) an appearance of P3HT vibronic bands in UV-Vis absorption spectra (b) an increase in diffraction peak of P3HT in XRD pattern. Figures (a) and (b) are respectively reproduced from references [116] with permission of AIP Publishing LLC and [238] with permission of Elsivier.

polymeric matrix and aggregate to form needle-like crystallites that can reach sizes up to a few microns under extensive thermal annealing. Upon moderate thermal annealing, P3HT and PC61 BM are subject to phase separation causing an increase in D and A domain sizes. Most studies report thermal annealing for a duration of 1 to 30 minutes at temperatures ranging from 110 o C - 160 o

C. Upon controlled thermal annealing, Jsc and FF were shown to significantly

increase resulting in PCEs of up to 5% [43, 115, 134, 160, 238]. A slow drying approach - or solvent annealing - was also used to control the morphology of P3HT/PC61 BM-films. This approach generally involves spin-casting the D/A for a short period of time so that the resulting film still contains solvents; the film can then be placed in a solvent saturated environment. In the saturated environment, the solvent evaporation is slowed compared with evaporation during conventional spin-casting. Similarly to what is observed in thermal annealing, the degree of P3HT crystallinity increases and high efficiencies are obtained upon solvent annealing [50, 78, 117]. This approach can be interpreted as an extension of the effects of high boiling point solvents described in section 2.1: during solvent annealing, the polymer chains are given more time to aggregate which results in an overall increase in domain sizes and degree of crystallinity [164, 211]. Post-processing steps can increase the efficiency of P3HT/PC61 BM-based OSCs. From a fundamental point of view, these approaches also provide insight into the 26

Formulation strategies for controlling BHJ morphology

relationship between active layer morphology and photovoltaic performance. In practice, however, thermal annealing is hardly compatible with flexible substrates that require low temperature processing conditions; overall the post-processing steps are difficult to implement in a large area high throughput line of fabrication without impacting the final cost of OSCs. Additionally, these approaches are inefficient for some low band gap polymers [44, 166]. These limitations motivate the need of a different strategy to control the BHJ morphology - specifically the use of processing additives in D/A formulations.

2.3

Use of processing additives in BHJ-OSCs

A processing additive refers to a solvent which is introduced in small proportion (generally a few volume %) into the host solvent used to solubilize the active materials. In 2006 - 2007, Peet et al. showed that introducing 1,8-octanedithiol (ODT) in a D/A solution blend increased the photocurrent and the PCE of OSCs using P3HT or PCPDTBT as D materials [169, 170]. For PCPDTBT/PC70 BMbased OSCs, the PCE was increased from 2.8% to 5.5%. This pioneering work represented an exciting discovery as processing additives were seen as a new method for improving the photovoltaic performance of OSCs in addition to the conventional post-processing steps. Further work led to two competing theories regarding the specific properties of what makes a solvent an efficient processing additive. In 2008, Yao et al. proposed that processing additives should possess:[234] (i) a lower vapor pressure than the host solvent and (ii) a lower solubility limit towards the fullerene derivative acceptor than that of the host solvent. They based their selection rules on the P3HT/PC61 BM system and suggested that during spin-casting, the processing additive and the host solvent evaporate with different rates because of the difference in vapor pressure. The host solvent, with a higher vapor pressure, would evaporate more quickly causing an increase in the additive concentration in the solvent mixture. As a consequence, the solvent quality towards PC61 BM would decrease, leading to early formation and precipitation of clusters. This mechanism is illustrated in Figure 2.4. With a smaller amount of PC61 BM in the solution, the P3HT chains would be able to 27

Formulation strategies for controlling BHJ morphology

self-organize more easily and form more crystalline domains at the origin of the improved active layer morphology.

Figure 2.4: Proposed model during spin-coating process. Black wire: P3HT polymer chain; big black dots: PC61 BM; blue dots: ODCB molecules; and red dots: ODT molecules. (a–c) correspond to three stages in the spin-coating process when ODCB is the sole solvent; (d–f) correspond to three stages in the spin-casting process when ODT is added in ODCB. Reproduced from reference [234] with permission of Wiley.

In the same year, Lee et al. proposed a different set of criteria for selecting processing additives, claiming that an ideal processing additive should have [114]: (i) a good solubility towards the fullerene derivative acceptor, (ii) a poor solubility towards the polymer and (iii) a boiling point higher than that of the host solvent. The mechanism proposed here is based on the fact that processing additives are good solvents for the fullerene derivatives and are poor solvents for the D (such as P3HT or PCDTBT). During the transition from the liquid to the solid states, the high boiling point of the solvent means that the fullerene derivatives remain longer in the solution state; this enables control of the phase separation between the polymer and the small molecule. Such a mechanism is depicted in Figure 2.5.

This mechanism is therefore based on the differential solubility of the additive towards the polymer and the fullerene derivative whereas the first mechanism accounts for the poorer solubility of the acceptor derivative in the additive compared with the host solvent. So far, however, no clear selection rules for selecting processing additives have emerged. 28

Formulation strategies for controlling BHJ morphology

Figure 2.5: Schematic of the role of the processing additive in the self-assembly of BHJ blend materials. Reprinted with permission from [114]. Copyright (2008) American Chemical Society.

Experimentally, other solvents were found to increase the PCE of OSCs: alkanedithiol with various alkane chain lengths [159, 169, 182], 1,8-diiodooctane (DIO) [114, 131], 1-chloronaphthalene (CN) [7, 11, 92, 147], nitrobenzene [47], 1-methyl-2-pyrrolidinone (NMP) [234] and 1,8-dichlorooctane [66, 114] among others [125, 128]. Figure 2.6 depicts the chemical structures of some of the processing additives found in literature. For the D/A system based on P3HT/PC61 BM, introducing nitrobenzene or alkanedithiols in the D/A solutions increased the PCEs up to 4% [149, 159]. These efficiencies are in the range of those normally obtained with post-processing steps [37, 159]. High efficiency P3HT/PC61 BM-based OSCs can therefore be fabricated without the use of post-processing steps, which is particularly appealing for the prospect of low fabrication cost and large area OSCs [58]. Literature shows that processing additives can increase the efficiency of OSCs in a wide range of D/A systems by optimizing the active layer morphology [63, 125, 171, 217]. Table 2.2 shows examples of some of these D/A systems along with the PCE values before and after introducing the processing additives. Based on these examples, it is clear that the use of processing additives represents a widely applicable and efficient approach to optimize a large range of D/A systems.

29

Formulation strategies for controlling BHJ morphology

Figure 2.6: Chemical structures of processing additives.

2.4

Context and objectives of the thesis

High-efficiency OSCs require a controlled and optimized BHJ morphology. Solvent choice in the D/A formulation is of major importance, but has a limited impact on the morphology due to the limited number of good solvents available for organic semiconductors. Also, upon spin-casting, the drying kinetics often dominate over the thermodynamic aspects of the D/A system, which leads to poor phase separation and subsequently low PCE in the case of P3HT/PC61 BM. Post-processing steps were successful in optimizing P3HT/PC61 BM-based OSCs, but were not efficient in optimizing several systems based on other low band gap polymers. Additionally, post-processing steps are not ideal when thinking towards flexible and low-cost OSCs. The use of processing additives appears to be a key approach to optimize D/A morphology: it is widely applicable and compatible with large area and high throughput fabrication of OSCs. While many studies have reported the success of processing additives in increasing the PCE of OSCs, fundamental aspects surrounding their use have not been addressed as successfully. So far, there has been no systematic approach to identify suitable processing additives for a given D/A system. Very few studies have addressed the issue of how to identify or select processing additives from the vast array of available

30

Formulation strategies for controlling BHJ morphology Table 2.2: Effects of processing additives on the PCE of several types of D/A OSCs. D/A systems1

Processing additive

PCE w/o

PCE with

Phase

Ref.

additive [%] additive [%] separation

ODT

0.6

2.6

increase

[37]

1,6-hexanedithiol

0.46

3.16

increase

[182]

Nitrobenzene

1.2

3.94

-

[149]

Dodecanedithiol

1.74

4.03

-

[159]

NMP

0.29

∼1.52

-

[234]

DIO

1.15

2.97

decrease

[70]

3.09

decrease

P3HT/PC61 BM

P3HT/IC70 BA

ODT PCDTBT/PC71 BM

DIO

4.89

5.91

increase

[131]

Alkanedithiol

2.8

5.5

increase

[35]

DIO

2.6

4.5

increase

[8]

ODT

3.35

4.5

increase

[114]

DIO

3.35

5.12

increase

[114]

1,8-dibromooctane

3.35

4.66

increase

[114]

4.62

increase

3.87

increase

3.45

increase

PCPDTBT/PC71 BM

DIO PCPDTBT/PC61 BM

ODT

1.68

1,8-dichlorooctane

[66]

PTB7/PC71 BM

DIO

3.92

7.4

decrease

[124]

DPPT/PC61 BM

DIO

0.6

3.4

decrease

[127]

DPPBT/PC61 BM

DIO

1.0

5.2

decrease

[127]

PDTSTPD/PC61 BM

DIO

1.0

7.3

decrease

[6, 46]

1 The 2 No

acronyms used for the D and A materials are defined in the Abbreviations section

exact PCE was reported

solvents. Consequently, their selection is generally the result of trial and error experiments. Comparing the different studies in the literature also shows that there is no clear consensus on the mechanistic role of processing additives in influencing and controlling the BHJ morphology. Their effects on D/A phase separation have been the subject of several studies. The results of such studies, summarized in 31

Formulation strategies for controlling BHJ morphology

Table 2.2, show that the effects of processing additives on phase separation vary with the type of D/A blend under investigation: in some cases they increase the phase separation while in other cases, they decrease it. In the case of D/A blends based on P3HT, the literature reports contradictory results regarding the effects of additives on phase separation. Another major aspect not considered in the literature is the effect of additives on the long-term stability of OSCs. As OSC stability is equally important as the PCE, investigating their effects on stability is of major importance. An in-depth understanding of the effects of processing additives is required to enable and accelerate the development of efficient D/A formulations. This thesis aims to carry out a comprehensive investigation of processing additives, addressing the following objectives: 1. To establish a predictive method for identifying processing additives for D/A systems with certain given properties. 2. To elucidate the mechanistic role of processing additives in influencing and controlling the BHJ morphology and subsequently, the OSC performance. 3. To determine the impacts of the processing additive approach on the stability of OSCs. In pursuit of these objectives, the study will primarily focus on the archetypical P3HT/PC61 BM system. The last chapter of this thesis deals with the effects of processing additives in systems using other types of D polymers.

32

Chapter 3

Experimental details

33

Experimental details

3.1

Fabrication and characterization of OSCs

This PhD thesis being done in the context of a co-tutelle program between the University of Waterloo and the University of Bordeaux, the experimental details differ from experiment to experiment. Experimental details are described according to the laboratory where the studies were conducted. Experiments reported in Chapters 4, 6 and 8 were performed at the University of Bordeaux laboratory (UB Lab). Experiments reported in Chapters 5 and 7 were performed at the University of Waterloo Laboratory (UW Lab).

3.1.1

Materials and substrates

The P3HT and PC61 BM were purchased from Solaris Chem Inc. and used as received. The P3HT has a molecular weight of 53 kDa and a polydispersity index of 1.49. The purity of PC61 BM is 99.74%. The PCDTBT is purchased from Solaris Chem Inc., its molecular weight is 85 kDa. The PDQT was synthesized in UW Lab according to the literature [123]. The average molecular weight is 21100 and the polydispersity index 2.72. The solvents: 1,2-dichlorobenzene, chlorobenzene were purchased from Sigma Aldrich and used as received. The processing additives: dimethyl phthalate, 1,8octanedithiol, 1,8-diiodooctane, 1-cyclohexyl-2-pyrrolidinone, tributyl-o-acetylcitrate were purchased from Sigma Aldrich and used as received. 1-chloronaphthalene was purchased from TCI Chemical N.V. and used as received. Zinc acetate dihydrate was purchased from Sigma Aldrich with a purity above 99.9%. At UB Lab, the ITO coated substrates used for device fabrication were purchased from Kintec. The resistance of the ITO layer is 10 Ω/square. The shadow masks used for device fabrication enable the fabrication of four OSCs, with a surface area of 8.6 mm2 , on each substrate. 34

Experimental details

At UW Lab, the ITO coated substrates were purchased from Luminescence Technology Corporation. The resistance of the ITO layer is 15 Ω/square. The shadow masks used for device fabrication enable the fabrication of three OSCs, with surface areas of 10 mm2 , 12.5 mm2 and 17.5 mm2 , on each substrate.

3.1.2

Active layer formulation

The organic semiconductors are weighed with the desired proportion in vials. Solvent mixtures are introduced to the organic semiconductors to solubilize them. The preparation of solvent mixtures differs as a function of the proportion of processing additive in the host solvent. For mixtures with a processing additive concentration below 0.8 vol%, the solvent mixture is prepared separately by blending and stirring the additive and the host solvent for 1 - 2 hours at ambient temperature. The prepared solvent mixture is further introduced in the vial containing the organic semiconducting materials. For solvent mixtures with a processing additive concentration above 0.8 vol%, the solvent and the processing additive are introduced directly with the desired concentration in the vial containing the organic semiconductors. The final mixture containing the solvent mixture and the organic semiconductors is first stirred for 15 - 20 minutes at 90 °C in order to ensure an efficient dissolution of the polymer and to avoid large aggregates. The mixture is then stirred at 50 °C. For P3HT/PC61 BM solutions, the mixtures are found to be stable up to at

least 6 weeks. Therefore P3HT/PC61 BM solutions were kept and re-used several times. For PCDTBT/PC61 BM and PDQT/PC61 BM solutions, the mixtures were stirred for a maximum period of 12 hours prior to use.

3.1.3

Substrate cleaning

The ITO coated substrates were systematically cleaned in an ultrasonic bath of acetone, ethanol and isopropanol for 15 minutes each. The substrates were then dried using a nitrogen gun and were treated in a UV-ozone oven (in UB Lab) or were treated with O2 Plasma (in UW Lab).

35

Experimental details

3.1.4

Fabrication of OSCs using a conventional architecture

For conventional OSCs, the cleaned ITO substrates were treated under UVozone for 20 minutes. On top of cleaned ITO coated substrates, a thin layer of PEDOT:PSS (Baytron P, Bayer AG/Germany) was spin-cast at 4000 rpm for 60 seconds in air. The sample is further dried at 110 °C in a vacuum oven for 20 minutes. The thickness of the PEDOT:PSS layer was determined to be ∼ 50 nm. The D/A solution is further deposited by spin-casting in a glovebox environment (O2 and H2 O levels < 0.1 ppm). Finally, calcium (20 nm) and aluminum (∼ 50 70 nm) were thermally evaporated under a secondary vacuum (10−6 mbar) onto the active layer through a shadow mask. Due to the low stability of conventional OSCs in atmosphere conditions, conventional OSCs are either measured in a glove box environment, or encapsulated (using a photo-sensitive epoxy glue to glue a glass substrate on top of the top electrode) prior to J –V characterizations measurement in air.

3.1.5

Fabrication of OSCs using an inverted architecture

For inverted OSCs, the cleaned ITO substrates were treated under UV-ozone for 10 minutes or O2 plasma for 2 minutes. A zinc acetate precursor solution is prepared by dissolving 196 mg of zinc acetate dihydrate in 6 mL of ethanol absolute. 54 µL of ethanolamine is then introduced. The solution is then stirred on a hotplate for 2 hours at 45 °C and subsequently filtered using a 0.45 µm cellulose acetate filter. On top of cleaned ITO coated substrates, the solution of zinc acetate precursor solution is spin-cast at 700 rpm for 60 seconds. The ZnO layer is further thermally annealed on a thermally controlled hot plate at 180 °C for 60 minutes. The D/A solution is further deposited by spin-casting in a glovebox environment (O2 and H2 O levels < 0.1 ppm). Finally, molybdenum oxide (7 nm) and silver (70 nm) were thermally evaporated under a secondary vacuum (10−6 mbar) onto the active layer through a shadow mask.

36

Experimental details

3.1.6

J –V characteristics measurements of OSCs

At UB Lab, J -V analysis was conducted using a K.H.S. SolarCelltest- 575 solar simulator with AM1.5G filters set at 100 mW.cm−2 with a calibrated radiometer (IL 1400BL). A LabVIEW-controlled Keithley 2400 SMU enabled the measurement of J -V curves. At UW Lab, the J –V analysis was conducted on ABET technologies Sun 2000 Solar Simulator with AM1.5G filters set at 100 mW.cm−2 . A LabVIEW-controlled Keithley 2400 SMU enabled the measurement of J -V curves. Jsc , Voc , FF and the PCE were extracted from the Labview softwares used for the J –V characterizations. The shunt resistance (Rsh ) and the series resistance (Rs ) can also be extracted from the J-V characteristics. Three regions of the J-V characteritsics can be defined as a function of the value of the applied voltage: - At low voltage, the J-V characteristic is primarily determined by Rsh which can be determined from the slope of the J-V characteristic at V = 0 V: 1 dJ = ( )V =0 Rsh dV

(3.1)

- At intermediate voltage, the diode parameter dominates. - At higher voltage, the J-V characteristic is primarily determined by Rs . There are several methods for determining and fitting Rs . One of the simplest is to determine the slope of the J-V curve at high voltage. In the literature, the slope is found to be determined at different voltage values [45, 91, 205, 213]. It is however common to determine the series resistance at Voc in order to be close to the operating conditions:[94, 102] 1 dJ = ( )V =Voc Rs dV

37

(3.2)

Experimental details

3.2

Mobility measurements

In this thesis, the measurements of charge carrier mobilities are determined by two methods: the determination of charge carrier mobility in an organic thin film transistor (OTFT) configuration and in a single diode configuration.

3.2.1

Mobility measurement in OTFT configuration

An OTFT is a three-terminal device with a gate, a source and a drain. The current flowing between source and drain (IDS ) can be controlled by the applied gate voltage across a thin dielectric film. By applying a gate source voltage (VGS ) across the dielectric, a channel of charge carriers is induced in the semiconductor layer at the interface with the dielectric. This channel allows the drain current to flow through the semiconductor when another voltage (the drain source voltage VDS ) is applied between the drain and the source. If VGS is positive, negative charges will be attracted to the interface between the semiconductor and the dielectric, the channel is called n-type channel. On the contrary, when VGS is negative, positive charges are induced at the interface between semiconductor and dielectric and the channel is a p-type channel. Electron mobility can be extracted from the n-type OTFT, while hole mobility can be extracted from the p-type OTFT. The OTFT architecture used in this work is bottom gate, top contact as presented in Figure 3.1. The channel length (L) and channel width (W) are respectively 50 µm and 1000 µm. The substrates used are silicon wafer. Au

L

Au Organic semi-conductor

Gate dielectric

Au

W

Au

Gate Substrate

Figure 3.1: P-type bottom-gate, top-contact OTFT device architecture from a sideview (left) and from a top-view (right). L represents the channel length and W the channel width.

38

Experimental details

A layer of poly(1-vinyl-1,2,4-triazole) (PVT) is used as a dielectric. The PVT solution is prepared by dissolving PVT in ultra-pure water at a concentration of 7 weight%. The solution is filtered and spin-cast at 700 rpm for 60 seconds. The substrates are further dried at 85 °C for 120 min under a primary vacuum. The organic semiconductor materials to study are deposited on top of the dielectric. The deposition of the metals is performed by thermal evaporation. For p-type OTFTs, the electrode is gold. For n-type OTFTs, the electrode is aluminum. The electrical characterizations are performed in a glovebox environment using a probe station (SUSS Microtec). Micro probes are used to take the contacts. A Labview controlled Keithley 4200 SMU was used for the acquisition of the electrical characteristics. A VDS of +5 V is applied for n-type OTFTs and a VDS of -5 V is applied for p-type OTFTs. The mobility is extracted from the saturation regime where |VDS | > |VGS - Vth | > 0. In the saturation regime, the current is given by: IDS =

µCi W (VGS − Vth )2 2L

(3.3)

and the carrier field effect mobility is given by:

µsat

√ 2L ∂ IDS 2 = ( ) Ci W ∂VGS

(3.4)

where Vth is the threshold voltage, Ci the gate dielectric capacitance per unit area (=14.7 nF/cm2 ), µsat the mobility, W the channel width and L the channel length. Vth can be extracted from measurements in the saturation region by plotting √ IDS versus VGS and extrapolating to IDS = 0 [226].

3.2.2

Mobility measurement in single diode configuration

In the field of organic semiconductors, the extraction of the mobility using the space charge limited current (SCLC) has emerged as a common method to extract 39

Experimental details

the charge mobility of materials [110]. The SCLC method is applied for single charge carrier devices, so called hole only devices or electron only devices. The example of hole only devices is taken to describe the electrical characteristics of a single charge carrier diode. In hole only devices, holes are injected in abundance by one of the electrode. These charge carriers are not compensated by an equal density of electrons and build up in the semiconductor: a positive space charge is formed in the semiconductor. Further increase of the voltage leads to the spacecharge limited current, which is the maximum current that the semiconductor can sustain. Above a certain voltage, the current shows a quadratic behavior and the current density J can be characterized by the Child’s law (also called Mott-Gurney law): 9 V2 J = r 0 µ 3 8 L

(3.5)

where 0 is the permittivity of the vacuum, r the dielectric constant of the polymer (assumed to be 3, which is a commonly used value for conjugated polymers), µ the mobility, V the voltage drop across the device and L the thickness of the material under study. The previous equation assumes that the mobility is field independent. In order to consider a mobility that is field dependent, a modified equation is preferred (also called Murgatroyd equation, [137]) and J can be described by: r V 9 0 µ0 V 2 exp(0.89 ) J= 3 8 L E0 L

(3.6)

where µ0 is the zero-field mobility and E0 the characteristic field. Single charge carrier devices are fabricated and further electrically characterized in order to extract the dark J -V curves. By fitting the J -V curves with the equations 3.5 or 5.2, the mobility can be extracted. In the early years, the Child’s law was used to determine the mobility of single layers of materials such as (phenylene vinylene) derivatives [17, 137, 142], P3HT [203] and PC61 BM [145]. Later, the use of the SCLC method has been broadened to the determination of single charge carrier mobility of a material blend in a BHJ configuration [143]. In order to assess the mobility of a single charge carrier of a material in a BHJ configuration, the other carrier has to be blocked. For example, for the study of hole mobility, hole only devices have to be fabricated with appropriate electrodes 40

Experimental details

and electron blocking layer in order to inhibit the transport of electrons by the acceptor. Hole only devices have been fabricated with the following configuration: ITO / MoO3 /Active layer/MoO3 /Ag (Figure 3.2). Clean ITO substrates were treated under O2 plasma for 2 minutes. MoO3 (7 nm) is evaporated on the ITO substrate under a secondary vacuum. The D/A solution is further deposited by spincasting in a glovebox environment. Finally, MoO3 (7 nm) and silver (70 nm) were thermally evaporated under a secondary vacuum (10−6 mbar) onto the active layer through a shadow mask. ITO MoO3 P3HT MoO3 Ag

Xe-

Ag MoO3

2.3 eV

2.3 eV 3.2 eV

Active layer 4.7eV 5.3 eV

MoO3 Glass/ITO

5.1 eV

5.3 eV

4.7 eV

Figure 3.2: Hole only device structure.

3.3 3.3.1

Characterization of the BHJ morphology UV-Vis absorption spectroscopy

UV-Vis spectra are recorded using a SAFAS UVMC2 spectrophotometer at UB Lab and a Shimadzu UV-2501PC UV-Vis spectrophotometer at UW Lab.

3.3.2

X-ray diffraction measurements

The diffraction patterns obtained from the PDQT study were recorded at UB Lab using a Goniometer Pranatycal X’pert Pro. The diffraction patterns obtained from the P3HT/PC61 BM study were recorded at UW Lab using a Bruker D8 advance diffractometer. The XRD measurements were performed in a reflection mode using Cu Kα1 radiation at 1.5406 ˚ A. 41

Experimental details

The average size L of crystallites can be determined with the full width at half maximum of the diffraction peak using the Scherrer equation: L=

Kλ ∆cos(θ)

(3.7)

where K is the Scherrer’s constant which depends on the crystallite shape and size distribution [112].

3.3.3

Atomic force microscopy

AFM images of PDQT were performed in UB Lab using a Veeco Dimension 3100. AFM images of P3HT/PC61 BM-films were performed in UW Lab using a Dimension 3100 Scanning Probe Microscope.

3.3.4

Infrared absorption spectroscopy

PM-IRRAS spectra were recorded on a ThermoNicolet Nexus 670 FTIR spectrometer at a resolution of 4 cm−1 , by coadding several blocks of 1500 scans (30 minutes acquisition time). Experiments were performed at an incidence angle of 75 °using an external homemade goniometer reflection attachment, adding a ZnSe photoelastic modulator (PEM, Hinds Instruments, type III) after the polarizer [25]. The ATR spectra of the additives were recorded with a ThermoNicolet Nexus 670 FTIR spectrometer equipped with a liquid nitrogen cooled narrow-band mercury cadmium telluride (MCT) detector using a Silver-Gate (diamond crystal) ATR accessory (Specac). Each spectrum was obtained from the acquisition of 100 scans at a resolution of 4 cm−1 .

3.4

Photo-stability tests

Photo-degradation tests were carried out with white light provided by a 300 W halogen lamp. The distance between the lamp and the device was adjusted so that the light intensity was 100 mW.cm−2 . The temperature is kept below 32 °C 42

Experimental details

throughout the experiment using a fan to cool down the OSCs subject to light irradiation. The temperature of the OSCs were monitored with a k-type thermocouple and an Omega panel monitor. All OSCs were kept in inert conditions during light-irradiation and were placed in air during the measurements of the J-V characteristics.

43

Chapter 4

Determination of selection rules for processing additives

44

Determination of selection rules for processing additives

This work has been published in AdvenMat, 4 (3) 1-9, 2013. It has been adapted with permission from the publisher under license number 3479061354313. As noted in Chapter 2, processing additives have been widely used for increasing the efficiency of OSCs. However, very few studies have addressed the critical question of how to identify or select processing additives from the vast array of available solvents. In this chapter, guidelines for the selection of processing additives are developed for P3HT/PC61 BM-based OSCs. First, the properties of existing processing additives reported in the literature are analyzed. The novelty of the developed approach is to use the theory of the Hansen solubility parameters to analyze these processing additives. This theory is commonly used in the coating and chemical formulation industries to determine the solubility of compounds. Nevertheless, its use in the area of BHJ started only very recently and has so far been limited to the identification host solvents [161, 214], or to correlate the host solvent solubility properties with the active layer morphology [57] and with the performance of OSCs [136]. Here, the Hansen solubility parameters are used to define the interactions between solvent, processing additives and active materials.

4.1

The Hansen solubility parameters as a method for selecting processing additives

The solubility parameter (δ), first used by Hildebrand and Scott, was originally defined as the square root of the cohesive energy density (EC ) over the molar volume of the pure solvent (V): r δ=

Ec V

(4.1)

The Hansen theory is based on substituting the total cohesive energy by three components; ED , EP and EH which describe the energy involved in three principal types of interactions, respectively: (i) dispersion interactions, (ii) permanent dipolar - permanent dipolar molecular interactions and (iii) hydrogen bonding interactions [73]. The parameter δ can therefore be similarly substituted by three components that describe these three interactions: 45

Determination of selection rules for processing additives

EC = ED + EP + EH

(4.2)

2 2 δ 2 = δD + δP2 + δH

(4.3)

Where δD , δP and δH are the Hansen solubility parameters (HSPs). Graphically, every chemical compound can be represented by its position in a 3D space, the Hansen solubility space, with coordinates defined by the three solubility parameters. The solubility of a solute in a solvent is predicted from similarities in their interactions. Such similarity is quantified by the distance RA between the HSPs of the solvent (δD1 , δP 1 and δH1 ) and the HSPs of the solute (δD2 , δP 2 and δH2 ). The distance RA between them is calculated using the following equation: 2 RA = 4(δD1 − δD2 )2 + (δP 1 − δP 2 )2 + (δH1 − δH2 )2

(4.4)

In addition to δD2 , δP 2 and δH2 , a solute requires a boundary of solubility to define and differentiate between “sufficient” and “non-sufficient” interactions from a solubility standpoint. Therefore a solute is described as a sphere in the Hansen solubility space, the HSPs are the centre of the sphere and RO is the radius representing the boundary of solubility. The interactions between a solvent and a solute are considered to be strong only if the distance RA is smaller than the radius of the sphere RO . The relative energy difference (RED) can be used as a numerical parameter to compare RA and RO , and is defined as: RED = RA /RO

(4.5)

If the RED is higher than 1, the solvent is outside the solubility sphere of the solute and can be expected to be a bad solvent. On the other hand, if the RED is between 0 and 1, the solvent is inside the solubility sphere of the solute and is expected to be a good solvent. The HSPs of a wide range of solvents can be found in reference textbooks [73]. They have been calculated using either equations of state derived from statistical thermodynamics or using the group contribution method. The group contribution method predicts the HSPs of a compound by adding the solubility parameters of all the contributing chemical groups or 46

Determination of selection rules for processing additives

atoms. For a new solute, the HSPs can be also determined experimentally by performing solubility tests. The solute is mixed with a wide range of solvents with different known HSPs. The quality of interactions is scored for each solutesolvent combination. The results are recorded on the Hansen solubility space and are fitted to a sphere with all the good solvents inside the sphere and the poor solvents outside. The HSPs of the compound result from the fitting of the sphere. Figure 4.1 depicts the process of sphere fitting using the software HSPiP 3rd edition.

Good solvents Poor solvents

Fitted solubility sphere

Figure 4.1: HSPs diagrams showing the good and the poor solvents resulting from solubility tests and the fitted solubility sphere of the compound under study.

4.2

Determination of the HSPs of P3HT and PC61 BM.

The HSPs of P3HT and PC61 BM were already determined by other groups [57, 63, 135, 136]. By comparing the results, it was found that the values can vary depending on the specific material characteristics (molecular weight and polydispersity of the polymer) and the method used in the solubility tests. Therefore, instead of using HSP values extracted from the literature, the HSPs of P3HT and PC61 BM are determined experimentally. The HSPs were determined by performing solubility tests in which the materials are mixed with various solvents that cover the whole range of the Hansen solubility space. Fifty-four solvents and around ten solvent mixtures were used. In each test, 4 mg of material is diluted in 2 mL of each solvent to obtain a concentration of 2 mg.mL−1 . Each solution is stirred overnight at room temperature. The quality of solubility is 47

Determination of selection rules for processing additives

assessed by visual inspection and assigned a score of 1 for good solubility or 0 for poor solubility, depending on whether or not a solid residue can still be detected after the stirring step. These results are used to fit the solubility sphere of the material where all the good solvents (the ones scored 1) are inside the sphere and the bad solvents (the ones scored 0) are outside the sphere. As the data fitting may contain false negative and false positive errors (i.e bad solvents inside the solubility sphere and good solvents outside the solubility sphere, respectively), the fitting accuracy needs to be estimated. This is usually estimated using a quality of fit function of the form:

DAT A F IT = (A1 ∗ A2 ∗ . . . ∗ An )1/n

(4.6)

where n is the number of solvents used for the solubility tests and Ai is given by:

Ai = e−(Error

distancei )

(4.7)

The Error distancei represents the distance of the false positive and false negative solvents from the sphere boundary. The DATA FIT reaches 1 for an ideal fit without any erroneous solvents. The results of the fitting are summarized in Table 4.1. Figure 4.2 depicts the solubility spheres of P3HT and PC61 BM. Table 4.1: HSPs of P3HT and PC61 BM.

HSP[MPA1/2 ]

Materials δD

δP

δH

Ro

P3HT

19.05

3.3

2.8

3.9

PC61 BM

20.02

5.2

5.88

8.4

a

Errors

Fit

False negative

False positive

0.95a

2/64b

1/64c

0.97a

2/65b

0/65c

As determined by the HSPiP software using Equation 4.6.

b

Number of bad solvents inside the solubility sphere.

c

Number of good solvents outside the solubility sphere.

48

Determination of selection rules for processing additives

Figure 4.2: Solubility spheres of P3HT and PC61 BM.

4.3

HSPs of commonly used processing additives

Next, the solubility properties of some processing additives reported to improve the efficiency for P3HT/PC61 BM-based OSCs in the literature are examined: ODT, 1,6-hexanedithiol, 1,12-dodecanedithiol, di(ethylene glycol)-diethyl ether (DEGDE), n-methyl-2-pyrrolidinone (NMP) and 4-bromoanisole. With the exception of 4-bromoanisole, which HSPs need to be calculated using the group contribution method, the HSPs of all these processing additives can be obtained from the database supplied by Hansen et al. [73]. After determining the HSPs of the processing additives and the active materials, the RED values are calculated using the equations described above. The results are shown in Table 4.2. For 4-bromoanisole, ODT, hexanedithiol and dodecanethiol, the RED values are found to be < 1 with PC61 BM and > 1 with P3HT. This is in agreement with the conclusions of Lee et al. that a processing additive should be a good solvent of PC61 BM and a poor solvent of P3HT. In contrast, NMP and DEGDE exhibit RED values with PC61 BM that are close to 1. The solubility of PC61 BM in NMP and DEGDE is reported to be low (∼0.3 mg.mL−1 [234]) which may explain their higher RED values. It is also noteworthy to point that DEGDE and NMP seem less effective in improving the efficiency of OSCs in comparison to the other processing additives (PCE: 1.5 % in case of NMP or DEGDE versus 2.6 - 4.0% with the other processing additives) [130, 159, 182]. The observation that can be made from this study is that efficient processing additives possess 49

Determination of selection rules for processing additives

RED values with PC61 BM clearly below 1 and RED values with P3HT above 1. This indicates the effectiveness of using the HSPs as a tool to predict novel processing additives for P3HT/PC61 BM-based OSCs. Table 4.2: Characteristics of processing additives: HSPs, RED with P3HT and PC61 BM, reported PCEs. HSPs [MPa1/2 ]

Materials δD

δP

δH

RED with Ro

P3HT

PC61 BM

P3HT

19.05

3.3

2.8

3.9

ND

ND

PC61 BM

20.02

5.2

5.88

8.4

ND

ND

DEGDE

15.8

4.7

4.4

1.75

1.02

PCEs [%] without

with

additive

additive

0.29%

5 times highera) [27]

NMP

18

12.3

7.2

2.62

0.99

0.29%

5 times highera) [27]

4-Bromoanisole

19.8

7.7

7

1.61

0.33

1.65%

2.60%[32]

ODT

17.4

7.3

5.2

1.47

0.68

0.46%

3.12%[19]

Hexanedithiol

17.4

7.6

6.4

1.67

0.69

0.46%

3.16%[19]

Dodecanethiol

17.3

5.7

4.1

1.14

0.68

1.74%

4.03%[26]

a)

4.4

No exact PCE is reported

Identification of novel processing additives

Based on the RED values above, the criteria that need to be satisfied by a processing additive for P3HT/PC61 BM-based OSCs are defined as follows: RED with PC61 BM < 0.9, RED with P3HT > 1.0 and boiling point 100 °C higher than that of the host solvent. The RED with PC61 BM is set to < 0.9 instead of < 1 in order to ensure selecting solvents in which PC61 BM solubility is sufficiently high. The third criterion is based on the general requirement that the boiling point of the processing additive must be higher than that of the host solvent, which in this case is ODCB (boiling point = 180 °C). A processing additive with a boiling point 100 °C higher provides a sufficient latitude for differential evaporation of the two solvents. Additionally, for safety concerns, only solvents that are not carcinogenic are considered. Applying the above selection criteria on an array of 723 solvents with known HSPs that are present in the Hansen 50

Determination of selection rules for processing additives

software database (HSPiP 3rd edition), three solvents were shown to satisfy the requirements. These are 1-cyclohexyl-2-pyrrolidinone (C-PYR), dimethyl phthalate (DPH) and tributyl o-acetylcitrate (TRIB). The molecular structures of the selected solvents are presented in Figure 4.3. As shown in Table 4.3 , they all have REDs with PC61 BM < 0.9 and REDs with P3HT > 1.0. Graphically, these three solvents are positioned inside the solubility sphere of PC61 BM and outside the sphere of P3HT, as depicted in Figure 4.4. Table 4.3: Characteristics of processing additives: HSPs, REDs with P3HT and PC61 BM and boiling points. Solvents

RED with

Boiling point

δD

δP

δH

18.2

6.8

6.5

1.38

0.48

306 o C

DPH

18.6 10.8

4.9

2.01

0.76

284 o C

TRIB

16.7

7.4

1.70

0.87

388 o C

C-PYR

N

HSPs [MPa1/2 ]

2.5

P3HT PC61 BM

O

(a) Figure 4.3: Molecular structures of: (a) C-PYR, (b) DPH and (c) TRIB.

For verification, the solubilities of PC61 BM and P3HT in these solvents are experimentally tested. P3HT was dissolved in processing additives with a concentration below 0.002 mg.mL−1 . For the specific case of DPH, P3HT was observed to float above the solvent, clearly indicating its non-solubility. For C-PYR and TRIB, UV-Vis absorption spectra of the solutions were measured and compared to the spectrum of a solution of P3HT in ODCB (concentration of 0.03 mg.mL−1 ) where the polymer is expected to be fully solubilized. The UV-Vis absorption spectra of P3HT in ODCB showed a unique absorption peak at 465 nm whereas the UV-Vis absorption spectra of P3HT in C-PYR and TRIB both showed a broad absorption peak extending beyond 650 nm indicating the presence of P3HT aggregates. The UV-Vis spectra are shown in Figure 4.5. These results indicate that P3HT is not soluble at a concentration of 0.002 mg.mL−1 in C-PYR and TRIB. 51

Determination of selection rules for processing additives

Figure 4.4: Positions of C-PYR, DPH and TRIB in the Hansen solubility space with respect to the solubility spheres of P3HT and PC61 BM. S o lu tio n s :

A b s o r b a n c e [ a .u ]

0 .0 3 m g /m L in O D C B 0 .0 0 1 1 m g /m L in C - P Y R 0 .0 0 1 4 m g /m L in T R IB

0 .5

0 .0 3 0 0

4 0 0

5 0 0

6 0 0

7 0 0

W a v e le n g th [n m ]

Figure 4.5: UV-Vis absorption spectra of solutions of P3HT in ODCB, C-PYR and TRIB.

Several solutions of PC61 BM in CB were prepared in various concentrations ranging from 0.01 mg.mL−1 to 0.042 mg.mL−1 . The UV-Vis absorption was measured for each of the solutions in order to trace a calibration curve (Figure 4.6). The calibration curve enables the estimation of the concentration of PC61 BM in any solution based on optical absorbance data. Over-saturated solutions of PC61 BM in C-PYR, DPH and TRIB were prepared 52

Determination of selection rules for processing additives

2

Abosorbance [a.u.]

y = 41.677x R² = 0.9759 1.5

1 Series1 Linear (Series1) 0.5

0 0

0.01 0.02 0.03 0.04 Concentration of PCBM [mg.mL-1]

0.05

Figure 4.6: Calibration curves obtained from dissolving PC61 BM in CB with different concentrations.

and stirred overnight at 50 o C. After a centrifugation step (7000 rpm for 10 min), the supernatants were removed, transferred and further diluted. The supernatants were diluted 1600 to 2000 times for solutions of C-PYR, 80 to 200 times for DPH and 30 to 40 times for TRIB. The absorbance maxima in the UV-Vis spectra of these diluted solutions were measured and the concentrations of initial solutions were determined using the calibration curve. The solubility limits of PC61 BM in the three solvents are listed in Table 4.4. Table 4.4: Solubility of P3HT and PC61 BM. Solvents

Solubility limits [mg.mL−1 ] P3HT

PC61 BM

C-PYR

0. In the saturation regime, the current is given by: IDS =

µsat Ci W (VGS − Vth )2 2L

(6.1)

and the carrier field effect mobility is given by:

µsat

√ 2L ∂ IDS 2 ( = ) Ci W ∂VGS

(6.2)

where Vth is the threshold voltage, Ci the gate dielectric capacitance per unit area, µsat the mobility, W the channel width and L the channel length.

6.3.2

Mobility measurements of films prepared from ODT

N-type and p-type OTFTs are fabricated with P3HT/PC61 BM active layers prepared with various concentrations of ODT (from 0 to 2.4 vol%). The transfer characteristics of the OTFTs are shown in Figure 6.7. 80

Effect of device architecture on OSCs prepared with additives 1 4

C o n c e n tra tio n o f O D T : 0 v o l % 0 .4 v o l% 1 .6 v o l % 2 .4 v o l%

0 1 2

-2

1 0

-4

8

-8

C o n c o f O D 0 0 1 2

-1 0 -1 2 -1 4

e n T v .4 .6 .4

tra : o l% v o v o v o

tio n

Id s (n A )

Id s (n A )

-6

-2 5

-2 0

-1 5

-1 0

-5

4 2

l% l% 0

l%

-1 6 -3 0

6

-2

0

-5

0

5

1 0

1 5

2 0

2 5

3 0

3 5

V g (V )

V g (V )

(a)

(b)

Figure 6.7: Transfer characteristics of OTFTs prepared with various concentrations of ODT in: (a) p-type OTFTs and (b) n-type OTFTs.

In p-type OTFTs, the transfer characteristics show that higher IDS is obtained in films treated with ODT. In contrast, for the n-type OTFTs, no significant change is observed from the transfer characteristics, except those prepared with 1.6 vol% of ODT. The mobilities are then calculated for different values of VGS using Equation 6.2. The extracted hole and electron mobilities are plotted as a function of VGS -Vth in Figure 6.8a. For p-type OTFTs prepared from 0 and 0.4 vol% of ODT, the IDS in the transfer characteristics is so low that the calculated hole mobilities appear to be in the noise margin and can therefore not be considered reliable. For ODT concentrations ≥ 1.6 vol%, the hole mobility increases significantly. In order to evaluate clearly the dependence of electron and hole mobilities on ODT concentration, they are extracted and compared at VGS -Vth values of + (or - ) 2 V for electron (or hole) mobility. The mobility values are depicted as a function of ODT concentration in Figure 6.8b. Table 6.1 shows the corresponding mobility values as well as the ratio of hole mobility (µh+ ) over electron mobility (µe− ). Figure 6.8b clearly shows that hole mobility increases with increasing concentration of ODT while electron mobilities remain relatively unchanged upon the introduction of ODT. Films prepared with concentrations of ODT ≤ 1.6 vol% demonstrate a µh+ /µe− ratio < 1 indicating higher electron mobility. However, OTFTs with an ODT concentration of 2.4 vol%, exhibit a µh+ /µe− ratio of 7.05 indicating that additives cause the hole mobility to surpass that of electrons. 81

Effect of device architecture on OSCs prepared with additives 8 .0 x 1 0

-4

C o n c e n tra tio n o f O D T :

-1

-4

3 .0 x 1 0

-4

2 .0 x 1 0

-4

1 .0 x 1 0

-4

E le c tro n m o b ility H o le m o b ility

.V

-1

4 .0 x 1 0

M o b ility (c m

M o b ility (c m

2

2

.V

-4

-1

2 .0 x 1 0

-4

.s )

-4

o l% v o l% v o l% v o l%

-1

4 .0 x 1 0

0 v 0 .4 1 .6 2 .4

.s )

6 .0 x 1 0

-4

5 .0 x 1 0

0 .0 0 .0

-4

-3

-2

-1

0

1

2

3

4

0 .0

V g -V th (V )

0 .8

1 .6

2 .4

C o n c e n tra tio n o f O D T (v o l% )

(a)

(b)

Figure 6.8: (a) Mobility values of OTFTs prepared with various concentrations of ODT as a function of VGS -Vth : the left side of the graph depicts hole mobility and the right side the electron mobility. (b) Electron and hole mobilities at VGS -Vth of +2 V and -2 V respectively, as a function of ODT concentration. Table 6.1: Mobility of holes and electrons as a function of the concentration of ODT.

6.3.3

ODT ratio

N-TYPE

P-TYPE

µh+ /µe−

[vol%]

*10−4 [cm2 .V−1 .s−1 ]

*10−4 [cm2 .V−1 .s−1 ]

0

2.04 ±0.06

∼0

-

0.4

1.47 ±0.07

∼0

-

1.6

1.39 ±0.29

0.78 ±0.03

0.56

2.4

0.61 ±0.08

4.31 ±0.46

7.05

Mobility measurements of films prepared from DPH

Similarly to ODT, n-type and p-type OTFTs are fabricated with active layers prepared with concentrations of DPH ranging from 0 to 2.4 vol%. Figure 6.9 depicts the transfer characteristics of the n-type and p-type OTFTs. The transfer characteristics of p-type OTFTs, depicted in Figure 6.9a, also demonstrate higher IDS upon increasing DPH concentration. Next, the OTFT mobilities are calculated at different values of VGS using Equation 6.2. Figure 6.8a depicts the hole and electron mobilities as a function of VGS Vth for OTFTs prepared from various concentrations of DPH and Figure 6.8b shows the evolution of the electron and hole mobility extracted at VGS -Vth values of +2 V and -2 V respectively. Similarly to the case of ODT, the hole mobility 82

Effect of device architecture on OSCs prepared with additives

increases with increasing concentration of DPH. It is found here that the electron mobility decreases upon the introduction of DPH. The ratios µh+ /µe− , presented in Table 6.2, are shown to be < 1 in OTFTs prepared with DPH concentrations ≤ 0.8 vol%. This ratio increases and is > 2 for OTFTs prepared with DPH concentrations of 1.6 and 2.4 vol%. This indicates that the introduction of DPH also cause the hole mobility to surpass that of electrons, similar to that observed with ODT. 0

1 4

C o n c e n tra tio n o f D P H :

-2

1 2

-4 -6 -1 0 -1 2 -1 4 -1 6 -1 8 -2 0 -2 2 -2 4

C o n c e n tra tio n o f D P H : 0 v 0 .4 0 .8 1 .6 2 .4 -3 0

-2 5

-2 0

-1 5

-1 0

-5

o l% v o v o v o v o

8

Id s (n A )

Id s (n A )

0 v 0 .4 0 .8 1 .6 2 .4

1 0

-8

6

l%

4

l% 2

o l% v o v o v o v o

l% l% l% l%

l% l% 0 0 0

5

1 0

1 5

2 0

2 5

3 0

V g (V )

V g (V )

(a)

(b)

Figure 6.9: Transfer characteristics of OTFT prepared with various concentrations of DPH in: (a) p-type OTFTs and (b) n-type OTFTs. -4

-4

o l % v o l % v o l % v o l % v o l%

3 .0 x 1 0

-4

2 .0 x 1 0

-4

1 .0 x 1 0

-4

E le c tro n m o b ility H o le m o b ility

2 .0 x 1 0

M o b ility (c m

M o b ility (c m

2

2

.V

.V

-1

-1

4 .0 x 1 0

0 v 0 .4 0 .8 1 .6 2 .4

-1

-1

.s )

C o n c e n tra tio n o f D P H :

.s )

6 .0 x 1 0

-4

0 .0 0 .0

-4

-3

-2

-1

0

1

2

3

4

0 .0

V g -V th (V )

0 .8

1 .6

2 .4

C o n c e n tra tio n o f D P H (v o l% )

(a)

(b)

Figure 6.10: (a) Mobility values of OTFTs prepared with various concentrations of DPH as a function of VGS -Vth : the left side of the graph depicts hole mobility and the right side the electron mobility. (b) Electron and hole mobilities at VGS -Vth of +2 V and -2 V respectively, as a function of DPH concentration.

83

Effect of device architecture on OSCs prepared with additives Table 6.2: Mobility of holes and electrons as a function of the concentration of DPH. DPH ratio [vol%]

6.4

N-TYPE *10

−4

2

−1

[cm .V

µh+ /µe−

P-TYPE .s

−1

]

*10

−4

2

−1

[cm .V

.s

−1

]

0

2.51 ±0.12

∼0

-

0.4

0.95 ±0.19

∼0

-

0.8

0.40 ±0.00

0.10 ±0.03

0.25

1.6

0.35 ±0.12

0.93 ±0.04

2.63

2.4

0.23 ±0.02

2.04 ±0.17

9.02

Origins of additive dependence on OSCs architecture

In order to explain the higher performance obtained from an additive-treated OSC in an inverted architecture compared to a conventional one, the question of the vertical phase separation with a depleted polymer layer at the bottom was addressed. The results show that OTFTs prepared with processing additives exhibit an enhanced hole mobility compared the OTFTs without the additive. This result suggests that processing additives do not lead to any vertical phase separation that decreases the D concentration in the lower part of the film. In view of the results obtained in OTFT-mobility measurements, it is concluded that the difference between conventional and inverted architectures do not arise from vertical phase separation. This observation is in disagreement with a recent report by Wang et al. where several techniques (transmission electron microtomography, X-ray photoemission spectroscopy and dynamic secondary ion mass spectroscopy) were used to suggest that the introduction of ODT caused the formation of a P3HT-depleted layer at the bottom of the BHJ [216]. However, the OTFT-mobility measurements conducted here show that, even in the hypothetical case of vertical phase separation caused by additives, the electron and hole mobilities are not impacted by it. Ruling out vertical phase separation effects, the difference between conventional and inverted OSCs may instead be explained in terms of the location where the excitons are formed. In both architectures, the light is received by the OSC through the ITO substrate. Consequently, photon absorption is expected to occur predominantly in the bottom part of the active layer. As a result, charge carriers are also predominantly generated near the bottom. In a conventional 84

Effect of device architecture on OSCs prepared with additives

architecture, the electrons are collected by the top electrode. In this case, the photo-generated electrons have to traverse across the whole thickness of the organic layer to be collected by the top electrode. In contrast, holes travel a much shorter distance to reach the hole collecting electrode. Therefore, in this configuration, a high electron mobility is more favorable to account for the long distance that the electrons need to travel. In other words, a ratio µh+ /µe− < 1 is preferred in this configuration. Such a mobility ratio is obtained in films treated with low concentrations of additive. This, in turn, could explain the superior performance of conventional OSCs prepared with low concentrations of additive as depicted in Figure 6.2 and Figure 6.5. An explanatory scheme supporting this proposition is depicted in Figure 6.11. LiF/Al Favorable architecture

Hole Mobility Electron Mobility

-1

-4

CONVENTIONAL , PCE = 2.6 %

h+ 1 can be expected to be more favorable for a more balanced collection. As the introduction of processing additive increases the hole mobility, the inverted architecture is favorable here. An explanatory scheme supporting this proposition is depicted in Figure 6.12.

85

Effect of device architecture on OSCs prepared with additives

-3

8.0x10

-4

6.0x10

-4

4.0x10

-4

2.0x10

-4

h+ e-

ZnO/ITO

h+ >> e-

INVERTED, PCE = 2.9 %

2

-1

Favorable architecture

Hole mobility Electron mobility

-1

Mobility (cm .V .s )

MoO3/Ag 1.0x10

LiF/Al Unfavorable architecture

0.0 -12

-8

-4

0

4

8

12

eh+

Pedot:PSS/ITO CONVENTIONAL , PCE = 1.3 %

Vg-Vth (V)

Figure 6.12: A schematic description of hole and electron transport in OSC prepared with 2.4 vol% of additive and subsequent preferential architecture. OSCs with 2.4 vol% depict the case where hole mobility is higher than electron mobility.

To conclude, additive-treated OSCs were shown to perform better in inverted OSCs compared to conventional OSCs. The results of OTFT mobility measurements confirmed that vertical phase separation is not responsible for the differences obtained in conventional and inverted OSCs. Instead, the increasing hole mobility with additive concentration indicates that the difference arises from the location where the excitons are formed. Additionally, this analysis indicates that the ratio of electron and hole mobilities appear to play a crucial role in the choice of OSC architecture.

86

Chapter 7

The effects of processing additives on the stability of OSCs

87

The effects of processing additives on the stability of OSCs

In this chapter, the question of the effect of the active layer processing conditions on the stability of OSCs is addressed. Commercial application of OSCs is only viable when stability requirements are met. Therefore the research on OSCs cannot be only focused on higher efficiencies, but also on the stability. Much research has focused on several stability related topics that include the identification of degradation factors and mechanisms, and routes for stability improvement. Therefore, before addressing the question of the effect of processing additives on the OSC stability, it is prudent to provide first a brief background on the stability issues of OSCs.

7.1

7.1.1

Background on the stability issues of OSCs

Stability of organic semiconductors

Organic semiconducting materials are known for their relatively poor stability to ambient conditions. Pure P3HT was shown to be susceptible to photo-bleaching in the presence of oxygen in the solution state [4], as well as in the solid state [83, 139, 140]. In the solid state, the degradation occurs by radical oxidation that first occurs at the side chains and further leads to the degradation of the polythiophene ring causing a loss of π-conjugation. Such degradation appears in the form of a decrease in the polymer optical absorption in the UV-Vis absorption spectrum. The photo-oxidation is accelerated in the presence of humidity, light and high temperature [83]. Aside from the light-induced changes in the presence of oxygen, P3HT is also subject to degradation under light irradiation in the absence of oxygen, a phenomenon called photolysis [138]. It is noteworthy to point out that the degradation rate caused by photolysis is much slower. Manceau et al. showed that light irradiation of P3HT in inert atmosphere caused its absorbance to decrease by 20% within 10 000 hours, whereas the same happens in only 20 hours in the presence of oxygen [138].

88

The effects of processing additives on the stability of OSCs

Despite the severe degradation behavior observed in neat materials, blends of D/A generally exhibit less degradation. The introduction of fullerene derivatives was shown to slow down the rate of photo-oxidation and photolysis of πconjugated polymers [30, 155, 184]. Figure 7.1 compares the UV-Vis absorption spectra of neat polymer and of polymer/PC61 BM blends under light irradiation in the presence or absence of oxygen.

(a)

(b)

Figure 7.1: (a) Normalized UV-Vis absorption (at 500 nm) of MDMO-PPV (4), MDMO-PPV/PC61 BM ( ) and normalized UV-Vis absorption (at 520 nm) of P3HT (•) and P3HT/PC61 BM () samples during photo-oxidation. (b) Normalized UV-Vis absorption of MDMO-PPV/PC61 BM (at 500 nm ( )) and P3HT/PC61 BM (at 520 nm ()) samples during photolysis. Reproduced from reference [180] with permission of Elsivier.

The comparison demonstrates that the introduction of PC61 BM reduces significantly the photo-bleaching of P3HT in the presence of oxygen and suppresses it almost completely in the absence of oxygen [180]. In inert atmosphere, the optical properties of P3HT were shown to be stable for periods as long as 5000 hours. The enhanced photo-stability of blends was attributed to radical scavenging property of PC61 BM and its ability to quench the P3HT singlet state [138]. Thermal stability represents a severe issue in the stability of D/A blends, mainly due to the tendency of PC61 BM to form micrometer size crystals. Such crystals can be observed in optical microscopy, as depicted in the Figure 7.2 which shows a thermally annealed P3HT/PC61 BM films exhibiting PC61 BM crystals.

89

The effects of processing additives on the stability of OSCs

Figure 7.2: Optical microscopy images of P3HT/PC61 BM layers before and after being thermally annealed at 150o C for 5 hours or 24 hours. Reproduced from reference [222] with permission of Wiley.

7.1.2

Stability of OSCs

The factors that cause degradation at the material level such as oxygen and light irradiation subsequently cause significant failure at the device level. Additionally, the OSCs under usage conditions can be subject to several other degradation mechanisms such as electrical stress and mechanical stress [23]. The mechanisms of degradation can be categorized in three types:[65] 1. Degradation of D and A materials, 2. Morphological changes in the active layer, 3. Interfacial degradation. Oxygen was shown to play a key role in degradation mechanisms (1) and (3). The effects of oxygen on mechanism (1) were discussed previously. Additionally, oxygen induces several types of interfacial degradation mechanism especially in conventional OSCs [104, 219]. Inverted OSCs in which holes are collected by the top electrode are more stable, owing in part, to the higher work function of the top electrode [49, 132]. Nevertheless, even in inverted OSCs, oxygen remains one the major factors of degradation because of the severe damages caused to the active layer [188]. In order to prevent deterioration from oxygen and humidity, much work has been done on the use of protection barriers and as a result, OSCs protected with barriers could demonstrate stabilities of several years [74, 105, 108]. To conclude, the issue of oxygen can then (at least partially) be solved by choosing an appropriate device architecture as well as using barriers to encapsulate OSCs.

90

The effects of processing additives on the stability of OSCs

In the absence of oxygen, light irradiation only does not play a major role on mechanisms 1 and 2. However, several studies did reveal the dependence of the photo-stability of OSCs on the types of interlayers suggesting the presence of photo-degradation at the interfaces (mechanism 3) [212, 225]. As reported in this brief literature review, each component of an OSC is subject to degradation: the electrodes, the interface between active layer and the contacts, and the organic semiconducting materials. These components have to be chosen carefully in order to limit OSCs degradation. On the other hand, the dependence of OSCs stability on the active layer morphology - and subsequently the processing conditions - has not yet been addressed adequately. In this chapter, the photo-stability of OSCs that are thermally annealed is compared to the photo-stability of OSCs fabricated using additives. Stability tests under light irradiation in air and in inert atmosphere are performed and compared. In order to limit thermal degradation of OSCs, the temperature of the substrates is kept below 32°C during light irradiation.

7.2

Traces of processing additives in the active layer

The boiling points of the processing additives are relatively high (C-PYR: 306 °C, DPH: 284 °C, ODT: 270 °C and TRIB: 388 °C). Therefore, traces of them

can be expected to be present in the active layer and to subsequently alter the stability of OSCs. In order to detect the presence of processing additive in the active layer, infrared (IR) absorption spectra of the active layers are characterized with Polarization Modulation-Infrared Reflection Absorption Spectroscopy (PM-IRRAS). The PM-IRRAS spectrum is compared to that of the pure additive determined by Attenuated Total Reflectance (ATR) Spectroscopy to detect whether peaks related to the additive are present in the PM-IRRAS spectrum. For each additive, two sets of P3HT/PC61 BM films are investigated: one set consists of as cast-films and a second set consists of films that were placed under vacuum at a pressure of ∼ 5x10−6 mbar, which reproduces the vacuum step caused by thermal evaporation of top contacts. Figure 7.3 depicts a scheme explaining the tests. The films of P3HT/PC61 BM are spin-cast from solutions containing 1.6 vol% of 91

The effects of processing additives on the stability of OSCs

processing additives on ITO substrates coated with ZnO. A P3HT/PC61 BM-film spin-cast without processing additive serves as a control film. Set 1 : as-cast PM-IRRAS spectrum

Spin-casting Solvent + additive

Comparison with ATR spectrum of Additive

Set 2 : placed under vacuum

Figure 7.3: Scheme of the procedure for the detection of processing additive in P3HT/PC61 BM films.

ˆ PM-IRRAS spectra of C-PYR-treated active layers

As depicted in the ATR spectrum of C-PYR in Figure 7.4a, C-PYR exhibits an intense absorption peak at 1683 cm−1 that accounts for the C=O stretching mode. Figure 7.4b shows the PM-IRRAS spectra of P3HT/PC61 BM films, prepared with and without C-PYR, in the 1600 – 1900 cm−1 region. The peak at 1683 cm−1 is absent in the control active layer but is detected in the films prepared with C-PYR. This suggests that, after spin-casting, residual C-PYR is present in the active layer. When the film is placed under vacuum, the intensity of the absorption peak at 1683 cm−1 decreases and is almost not detectable anymore. Therefore, the vacuum is effective in reducing the amount of C-PYR in the active layer. ˆ PM-IRRAS spectra of DPH-treated active layers

The ATR spectrum of DPH (Figure 7.5a) presents no absorption peak that can be clearly discriminated from the PM-IRRAS absorption spectra of P3HT/PC61 BM films (Figure 7.5b). However, the absorption peak corresponding to the C=O in DPH appears at lower wavelength compared to the C=O peak in P3HT/PC61 BM films. This difference causes a broadening of the peak in DPH-treated active layers (Figure 7.5b). In the spectrum of the control active layer, the FWHM of this peak is 17.1 cm−1 and it increases up to 19.6 cm−1 for DPH-treated films. 92

The effects of processing additives on the stability of OSCs

0 .0 4

1 -[R p (d )/R p (0 )]

A b s o rb a n c e [a .u .]

0 .0 4

0 .0 2

C o n tro A c tiv e A c tiv e C -P Y R

l a c tiv la y e r la y e r (A T R

e la y e r w ith C P Y R _ a s c a s t w ith C P Y R + v a c u u m )

-1

0 .0 5

1 6 8 3 c m C = O s tr e tc h in g

C -P Y R

0 .0 3

0 .0 2

0 .0 1

0 .0 0 3 5 0 0

3 0 0 0

2 5 0 0

2 0 0 0

W a v e n u m b e r (c m

1 5 0 0 -1

0 .0 0 1 9 0 0

1 0 0 0

1 8 5 0

)

1 8 0 0

1 7 5 0

W a v e n u m b e r (c m

(a)

1 7 0 0 -1

1 6 5 0

1 6 0 0

)

(b)

Figure 7.4: (a) ATR spectrum of C-PYR, (b) Zoomed-in PM-IRRAS spectra of P3HT/PC61 BM films with and without C-PYR (region 1900 - 1600 cm−1 ).

This suggests that there is a contribution of the C=O from DPH that is remaining in the active layer. The vacuum step reduces the FWHM to 17.7 cm−1 . This shows that the vacuum step is effective in removing DPH, but traces of it are still detectable in the active layer. -1

0 .0 5

1 -[R p (d )/R p (0 )]

A b s o rb a n c e [a .u .]

0 .0 4

C o n tr o l a c tiv e la y e r A c tiv e la y e r w ith D P H _ a s c a s t A c tiv e la y e r w ith D P H + v a c u u m D P H (A T R )

0 .0 2

1 7 2 9 c m C = O s tr e tc h in g

0 .0 6

1 7 4 0 c m

D P H

-1

0 .0 7

0 .0 4

0 .0 3

0 .0 2

0 .0 1

0 .0 0 3 5 0 0

3 0 0 0

2 5 0 0

2 0 0 0

W a v e n u m b e r (c m

1 5 0 0 -1

0 .0 0 1 8 2 0

1 0 0 0

1 8 0 0

)

1 7 8 0

1 7 6 0

W a v e n u m b e r (c m

(a)

1 7 4 0 -1

1 7 2 0

1 7 0 0

)

(b)

Figure 7.5: (a) ATR spectrum of DPH, (b) Zoomed-in PM-IRRAS spectra of P3HT/PC61 BM films with and without DPH (region 1820 - 1700 cm−1 ).

ˆ PM-IRRAS spectra of TRIB-treated active layers

As depicted in the ATR spectrum of TRIB in Figure 7.6a, the additive exhibits an intense absorption peak at 1740 cm−1 , which remains visible in the PM-IRRAS spectra of TRIB-treated films (Figure 7.6b). In the active layer prepared with 93

The effects of processing additives on the stability of OSCs

TRIB, the intensity of the peak at 1740 cm−1 is four times more intense than in a control film. The vacuum step reduces the intensity to 41% of the original signal before the vacuum step. Other peaks characteristic for the ATR spectrum of TRIB (at 959 cm−1 and 1076 cm−1 ) appear in films prepared with TRIB. The intensity of these peaks decreases after the vacuum step. To conclude, TRIB remains in P3HT/PC61 BM film after spin-casting and after the vacuum step. 0 .1 4

1 7 4 0 c m

C o A c A c T R

-1

0 .1 4

T R IB

0 .1 2

n tro tiv e tiv e IB (

l a c tiv e la y e r la y e r w ith T R IB _ a s c a s t la y e r w ith T R IB + v a c u u m A T R )

0 .1 2

0 .0 6

-1

-1

0 .0 8 0 .0 6

9 5 9 c m

0 .0 8

1 0 7 6 c m

1 -[R p (d )/R p (0 )]

A b s o rb a n c e [a .u .]

0 .1 0 0 .1 0

0 .0 4

0 .0 4

0 .0 2

0 .0 2

0 .0 0 2 0 0 0

0 .0 0 3 0 0 0

2 5 0 0

2 0 0 0

W a v e n u m b e r (c m

1 5 0 0 -1

1 0 0 0

1 8 0 0

1 6 0 0

1 4 0 0

1 2 0 0

W a v e n u m b e r (c m )

(a)

1 0 0 0 -1

)

(b)

Figure 7.6: (a) ATR spectrum of TRIB, (b) Zoomed-in spectra of the region 1800 1700 cm−1 of PM-IRRAS spectra of P3HT/PC61 BM films with and without TRIB.

94

8 0 0

The effects of processing additives on the stability of OSCs ˆ PM-IRRAS spectra of ODT-treated active layers

The peak at 2560 cm−1 in the ATR spectrum of ODT, attributed to S-H, is absent in the PM-IRRAS spectra of ODT-treated films shown in Figure 7.7. Overall, as cast-films and films placed under vacuum prepared from ODT show no significant difference compared to films prepared without additive. This observation suggest that the amount of residual ODT in P3HT/PC61 BM-films is negligeable. 0 .4 0

0 .0 5

C o n tr o l a c tiv e la y e r A c tiv e la y e r w ith O D T _ a s c a s t A c tiv e la y e r w ith O D T + v a c u u m

O D T

0 .3 5

0 .0 4

0 .2 5

1 -[R p (d )/R p (0 )]

A b s o rb a n c e [a .u .]

0 .3 0

0 .2 0 0 .1 5 0 .1 0

0 .0 3

0 .0 2

0 .0 1

0 .0 5 0 .0 0 3 5 0 0

0 .0 0 3 0 0 0

2 5 0 0

2 0 0 0

W a v e n u m b e r (c m

1 5 0 0 -1

1 0 0 0

3 5 0 0

)

3 0 0 0

2 5 0 0

2 0 0 0

W a v e n u m b e r (c m

(a)

1 5 0 0 -1

1 0 0 0

)

(b)

Figure 7.7: (a) ATR spectrum of ODT, (b) PM-IRRAS spectra of P3HT/PC61 BM films with and without ODT.

To conclude, the vacuum step was shown to be effective in reducing the amount of additive in as-cast films. Traces of DPH and TRIB are detected in the active layer even after the vacuum step while C-PYR and ODT are not.

7.3 7.3.1

Photo-stability tests on OSCs Photo-stability tests on OSCs in air

Photo-stability tests are performed on OSCs prepared with different types of active layers. Table 7.1 gives the initial photovoltaic performances (i.e. at t=0) of the OSCs under study. Attention was paid to fabricate OSCs with similar efficiencies (between 2.6% and 3.0%) in order to allow for comparison between them to yield accurate conclusions. 95

The effects of processing additives on the stability of OSCs Table 7.1: Initial electrical performances of OSCs with different types of active layer. Initial values

Thermal annealing

C-PYR

DPH

ODT

Jsc [mA.cm−2 ] Voc [V] FF

9.2

9.2

8.7

8.9

PCE [%] Rs [Ohm.cm2 ] Rsh [Ohm.cm2 ]

±0.6

±0.4

±0.7

±0.6

0.56

0.53

0.52

0.50

±0.03

±0.02

±0.01

±0.02

0.58

0.62

0.66

0.59

±0.03

±0.06

±0.02

±0.03

2.6

3.0

3.0

2.6

±0.4

±0.4

±0.3

±0.2

4.4

2.5

1.4

3.9

±2.3

±1.7

±0.4

±1.3

8008

8460

8356

7192

±1771

±2268

±1680

±1987

The OSCs are prepared and exposed to light irradiation in air. OSCs containing processing additives are exposed to light irradiation for a period of 252 hours while thermally annealed OSCs are exposed for only 90 hours because of the low performance obtained after this period. The J -V curves were measured at several intervals within the duration of the light irradiation. Table 7.2 indicates the normalized electrical parameters of OSCs after light irradiation and Figure 7.8 depicts the evolution of the photovoltaic parameters as a function of irradiation time. The normalized parameters correspond to the percentage of initial value (at t=0) measured before the prolonged light exposure. For all OSCs, the FF is the parameter that is the most impacted by the degradation. The decrease in FF is likely to be linked to the significant increase in Rs upon light irradiation. The increase in Rs suggests either an increase in the internal resistance of the active layer or an increase in the resistance at the contacts. The photo-stability of OSCs differs significantly as a function of the active layer processing conditions. The thermally annealed OSC is the least photo-stable with a PCE of 1.8% of the initial value after 90 hours of light irradiation. On the other hand, ODT-treated OSCs present the best photo-stability with a PCE remaining at 36.2% of the initial value after 292 hours of light irradiation. The most striking difference between thermally annealed and ODT-treated OSC is the 96

The effects of processing additives on the stability of OSCs Table 7.2: Normalized electrical performance after light irradiation in air: 252 hours of light irradiation for OSCs with processing additives, 90 hours for thermally annealed OSCs. Normalized values [%]

Thermal annealing

Jsc

C-PYR

DPH

ODT

49.1

63.2

59.4

85.7

Voc

16.7

55.3

48.3

81.5

FF

22.3

33.2

13.0

51.8

PCE

1.8

11.8

3.7

36.2

Rs

153.8

1168.8

2642.7

297.3

Rsh

6.4

34.1

60.7

77.0

Voc . For thermally annealed OSCs, the Voc decreases significantly with increasing irradiation time, whereas ODT-treated OSCs demonstrate less than 15% loss in Voc after the total irradiation time. OSCs with C-PYR and DPH-treated active layers present intermediate stabilities as depicted in Figure 7.8.

7.3.2

Photo-stability tests on OSCs in inert atmosphere

OSCs prepared in conditions identical to those previously described are fabricated and irradiated for a period of 393 hours in inert atmosphere. The J -V curves were measured at several intervals within the duration of the light irradiation. Table 7.3 reports the normalized photovoltaic parameters of OSCs after 393 hours of light irradiation and Figure 7.9 depicts the evolution of the photovoltaic parameters as a function of the time of light irradiation. The results depicted in Figure 7.9 show that OSCs photo-stability in inert atmosphere is significantly higher than that in air. This is expected as the inert atmosphere substantially reduces degradation due to photo-oxidation. Interestingly, the dependence of the photo-stability on the processing conditions follows the same trend as observed in air: thermally annealed OSCs exhibit the poorest photo-stability whereas ODT-treated OSCs exhibit the highest photostability. OSCs with C-PYR and DPH exhibit intermediate stabilities. Similarly, the Voc is the most discriminant parameter: the thermally annealed OSC has its 97

The effects of processing additives on the stability of OSCs T h e rm a l a n n e a lin g C -P Y R D P H O D T

[% ] N o rm a liz e d v a lu e o f V

N o rm a liz e d v a lu e o f J

8 0

o c

8 0

1 0 0

s c

[% ]

1 0 0

6 0

4 0

2 0

6 0

4 0

2 0

(a ) 0

(b ) 5 0

1 0 0

1 5 0

2 0 0

2 5 0

0

5 0

T im e o f d e g re d a tio n (a ir) [h o u rs ]

1 1 0

1 0 0

1 5 0

2 0 0

2 5 0

T im e o f d e g re d a tio n (a ir) [h o u rs ] 1 0 0

N o rm a liz e d v a lu e o f P C E [% ]

N o rm a liz e d v a lu e o f F F [% ]

1 0 0 9 0 8 0 7 0 6 0 5 0

(c ) 0

8 0

6 0

4 0

3 6 .2 %

2 0 1 1 .8 %

(d ) 0

5 0

1 0 0

1 5 0

2 0 0

0

2 5 0

5 0

T im e o f d e g re d a tio n (a ir) [h o u rs ]

1 5 0

2 0 0

2 5 0

1 2 0

[% ]

1 0 0

s h u n t

2 0 0 0

N o rm a liz e d v a lu e o f R

[% ] s e rie

1 0 0

T im e o f d e g re d a tio n (a ir) [h o u rs ]

2 5 0 0

N o rm a liz e d v a lu e o f R

3 .7 %

1 .8 %

1 5 0 0

1 0 0 0

5 0 0

8 0

6 0

4 0

2 0

(f)

(e ) 0

5 0

1 0 0

1 5 0

2 0 0

0

2 5 0

5 0

1 0 0

1 5 0

2 0 0

2 5 0

T im e o f d e g re d a tio n (a ir) [h o u rs ]

T im e o f d e g re d a tio n (a ir) [h o u rs ]

Figure 7.8: Normalized electrical parameters of OSCs subjected to light irradiation in air: (a) Jsc , (b) Voc , (c) FF, (d) PCE, (e) Rs and (f) Rsh .

Voc reduced to 79.2% after 393 hours of light irradiation, whereas the Voc of ODT-treated OSC remains almost unchanged (less than 5 % decrease from the initial value). Along the same line, the decrease in FF is the most significant in thermally-annealed OSC, whereas irradiated ODT-treated OSC demonstrate a FF maintained at 94.4% of the initial value after the total irradiation time. 98

The effects of processing additives on the stability of OSCs Table 7.3: Normalized photovoltaic parameters of OSCs after 393 hours of light irradiation in inert atmosphere. Normalized value [%]

Thermal annealing

Jsc

C-PYR

DPH

ODT

75.4

66.8

67.9

78.9

Voc

79.2

91.6

78.3

95.6

FF

52.9

73.7

67.2

94.4

PCE

32.0

44.9

36.0

71.2

Rs

128.2

145.1

402.2

119.1

Rsh

33.9

52.8

50.0

77.3

Overall, ODT-treated OSCs are the most photo-stable OSCs with a final PCE of 71.2% of the initial value while the PCE of thermally annealed OSCs is reduced to 32.0% of the initial value. To conclude, in air and in inert atmosphere, processing conditions have an influence on the photo-stability of OSCs. It is noteworthy to remind here that DPH-treated active layers were shown to retain some additive. However the photo-stability tests show that DPH-treated OSCs have a higher stability than thermally annealed OSCs. This suggests that the remaining additive does not play a major role in OSC photo-degradation. The following sections aim at identifying the origins of the photo-stability dependence on the processing conditions.

99

The effects of processing additives on the stability of OSCs

1 1 0

1 1 0

s c

9 0

N o rm a liz e d v a lu e o f V

N o rm a liz e d v a lu e o f J

1 0 0 9 0

o c

[% ]

1 0 0

[% ]

T h e rm a l a n n e a lin g C -P Y R D P H O D T

8 0

7 0

(a )

8 0 7 0 6 0 5 0

(b )

4 0

6 0 0

5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

4 0 0

0

5 0

T im e o f d e g re d a tio n (a ir) [h o u rs ]

1 1 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

4 0 0

T im e o f d e g re d a tio n (a ir) [h o u rs ]

N o rm a liz e d v a lu e o f P C E [% ]

N o rm a liz e d v a lu e o f F F [% ]

1 0 0

1 0 0

9 0

8 0

7 0

8 0 7 1 .2 %

6 0

4 5 .0 %

4 0 3 6 .0 % 3 2 .0 %

(c ) 0

(e ) 5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

2 0

4 0 0

0

T im e o f d e g re d a tio n (a ir) [h o u rs ]

6 0 0

5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

4 0 0

4 5 0

T im e o f d e g re d a tio n (a ir) [h o u rs ]

1 1 0

(e )

(f)

1 0 0

s h u n t

4 0 0

N o rm a liz e d v a lu e o f R

N o rm a liz e d v a lu e o f R

s e rie

[% ]

[% ]

5 0 0

3 0 0

2 0 0

1 0 0

9 0 8 0 7 0 6 0 5 0 4 0 3 0

0

5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

4 0 0

0

T im e o f d e g re d a tio n (a ir) [h o u rs ]

5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

T im e o f d e g re d a tio n (a ir) [h o u rs ]

Figure 7.9: Normalized electrical parameters of OSCs subjected to light irradiation in inert atmosphere: (a) Jsc , (b) Voc , (c) FF, (d) PCE, (e) Rs and (f) Rsh .

100

4 0 0

The effects of processing additives on the stability of OSCs

7.4

UV-Vis absorption spectroscopy of light-irradiated active layers

To investigate the effects of light irradiation on the optical properties of P3HT/PC61 BMfilms, the UV-Vis absorption spectra of active layer films were measured after being subjected to light irradiation for 60 hours in air. The spectra are shown in Figure 7.10. T h e r m a lly a n n e a le d

0 .2

(a )

0 .4

0 .2

0 .0 4 0 0

5 0 0

6 0 0

F re sh 6 0 h o u rs o f irra d ia tio n

0 .6

A b s o r b a n c e [ a .u ]

A b s o r b a n c e [ a .u ]

0 .4

0 .0

C -P Y R

F re sh 6 0 h o u rs o f irra d ia tio n

0 .6

(b )

7 0 0

4 0 0

W a v e le n g th [n m ] F re sh 6 0 h o u rs o f irra d ia tio n

D P H

0 .4

0 .2

0 .0

(c ) 5 0 0

6 0 0

7 0 0

0 .4

0 .2

0 .0 4 0 0

6 0 0

F re sh 6 0 h o u rs o f irra d ia tio n

O D T 0 .6

A b s o r b a n c e [ a .u ]

A b s o r b a n c e [ a .u ]

0 .6

5 0 0

W a v e le n g th [n m ]

7 0 0

(d ) 4 0 0

W a v e le n g th [n m ]

5 0 0

6 0 0

7 0 0

W a v e le n g th [n m ]

Figure 7.10: UV-Vis absorption of P3HT/PC61 BM films before and after light irradiation for 60 hours in air for (a) Thermally annealed, (b) C-PYR, (c) DPH and (d) ODT-treated active layers.

In air, the intensity of the absorption band of P3HT decreases after light irradiation for all the types of active layers, indicating that photo-oxidation of P3HT occurs in all cases. The loss in optical density at 601 nm is calculated and recorded in Table 7.4. The decrease in optical density of ODT-treated active layers appears to be in the same range as that of C-PYR and DPH-treated active layers. The higher 101

The effects of processing additives on the stability of OSCs Table 7.4: Decrease in optical density measured at 601 nm. Types of active layers

Loss in optical density [%]

Thermally annealed

11.0

C-PYR

4.8

DPH

7.0

ODT

6.4

photo-stability of ODT-treated OSCs seems therefore to be independent of the optical density losses associated with photo-bleaching. The UV-Vis absorption spectra of light-irradiated active layers in inert atmosphere are depicted in Figure 7.11. T h e rm a lly a n n e a le d T h e rm a lly a n n e a le d - irra d ia te d

0 .4

0 .2

0 .0

(a )

0 .4

0 .2

0 .0 4 0 0

5 0 0

6 0 0

C -P Y R fre sh C -P Y R irra d ia te d

0 .6

A b s o r b a n c e [ a .u ]

A b s o r b a n c e [ a .u ]

0 .6

(b ) 4 0 0

7 0 0

D P H fre sh D P H irra d ia te d

0 .2

(c )

0 .4

0 .2

0 .0 4 0 0

5 0 0

6 0 0

7 0 0

O D T fre sh O D T irra d ia te d

0 .6

A b s o r b a n c e [ a .u ]

A b s o r b a n c e [ a .u ]

0 .4

0 .0

6 0 0

W a v e le n g th [n m ]

W a v e le n g th [n m ] 0 .6

5 0 0

7 0 0

(d ) 4 0 0

W a v e le n g th [n m ]

5 0 0

6 0 0

7 0 0

W a v e le n g th [n m ]

Figure 7.11: UV-Vis absorption spectra of P3HT/PC61 BM films before and after light irradiation for 60 hours in inert atmosphere for (a) Thermally annealed, (b) C-PYR, (c) DPH and (d) ODT-treated active layers.

102

The effects of processing additives on the stability of OSCs

In inert atmosphere, the absorption spectra do not exhibit significant change after 60 hours of light irradiation indicating that the π-conjugated system as well as the chemical structure of P3HT are not affected by light irradiation. This confirms that OSC photo-degradation in inert atmosphere does not arise from chemical photo-degradation of P3HT. The results in UV-Vis absorption spectroscopy do not explain the dependence of OSC photo-stability on processing conditions. Besides, the presence of traces of additive does not explain it neither. These two results suggest that the major origin of the photo-degradation is not the active layer. Therefore, the degradation at the interfaces (between the active layer and the top electrode or the bottom electrode) are investigated. As the trends in photo-stability are similar in air and in inert atmosphere, the following studies are carried out in inert atmosphere only.

7.5

Photo-stability study on the bottom interface

Photo-stability tests are carried out on OSCs containing a buffer interlayer which makes the bottom interface independent from the active layer processing conditions. The objective here is to verify if the photo-stability dependence on the processing conditions remains identical after the insertion of the buffer interlayer. To this end, a layer of C60 is introduced as an interlayer between ZnO and the active layer as depicted in Figure 7.12.

N2 Ag MoO3 Active Layer C60 ZnO Glass / ITO

Light Figure 7.12: Scheme of photo-stability tests on OSC with a C60 buffer interlayer.

The energy level diagram, shown in Figure 7.13, suggests that a C60 interlayer introduces a small energy barrier (0.4 eV) to the transport of electrons from the 103

The effects of processing additives on the stability of OSCs

active layer to the ZnO layer. Therefore, the photovoltaic performance of OSCs with a C60 interlayer needs to be optimized. ITO

ZnO

C60

P3HT MoO3 PCBM

Ag

2.3 eV

3.2 eV

4.7eV

3.9 eV

4.4 eV

4.3 eV 4.7 eV 5.1 eV

5.8 eV

5.3 eV

6.1 eV

7.7eV

Figure 7.13: Energy levels of OSCs containing C60 as an interlayer between ZnO and the active layer.

A series of OSCs containing C60 interlayers with different thicknesses are fabricated. Figure 7.14 depicts the evolution of the PCEs as a function of the thickness of C60 . The results show that a C60 interlayer with a thickness ≤ 10 nm does not alter significantly the PCE of the different types of OSCs whereas 20 nm of C60 decreases the PCE of thermally annealed OSC. This decrease is attributed to structural changes in the interlayer caused by the thermal annealing step. As the introduction of a C60 interlayer with a thickness ≤ 10 nm does not affect the photovoltaic performance of OSCs, they are suitable to be used for photo-stability tests. 3 .5 3 .0

P C E [% ]

2 .5 2 .0 1 .5

(

1 .0 0 .5

T h e rm a lly a n n e a le d ) C -P Y R ) O D T ) ( ( 0

5

1 0

1 5

2 0

T h ic k n e s s o f C 6 0 in te rla y e r [n m ] Figure 7.14: Effects of C60 thickness on the PCE of OSCs.

104

The effects of processing additives on the stability of OSCs

Photo-degradation tests in inert atmosphere are performed on OSCs containing a 10 nm interlayer of C60 . The OSCs are irradiated for 390 hours and the J V characteristics are measured at several intervals within the duration of light irradiation. Figure 7.15 depicts the evolution of PCE and Voc as a function of the time of light irradiation for OSCs containing or not a C60 interlayer. The results show that regardless of the type of processing conditions, OSCs containing a C60 interlayer photo-degrade following the same trend as OSCs without interlayer. The unchanged trend in photo-stability indicates that the changes in photo-stability are not due to variations at the bottom contact caused by the processing additives. 1 0 0

9 0

N o rm a liz e d P C E [% ]

N o rm a liz e d V o c [% ]

1 0 0

8 0

7 0

6 0

(

T A /n o ) ( C -P Y R /n o ) ( O D T /n o ) ( (

5 0

( 0

5 0

1 0 0

T A / 1 0 n m ) C -P Y R /1 0 n m ) O D T /1 0 n m )

1 5 0

2 0 0

2 5 0

3 0 0

8 0

6 0

4 0

(

T A /n o ) ( C -P Y R /n o ) ( O D T /n o ) ( ( (

2 0 3 5 0

4 0 0

Irra d ia tio n tim e [h o u rs ]

0

5 0

1 0 0

T A / 1 0 n m ) C -P Y R /1 0 n m ) O D T /1 0 n m )

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

4 0 0

Irra d ia tio n tim e [h o u rs ]

(a)

(b)

Figure 7.15: Evolution of normalized (a) Voc and (b) PCE as a function of irradiation time for OSCs without (solid line) and with (dashed line) a C60 interlayer.

7.6 7.6.1

Photo-stability study on the top interface Photo-stability tests on OSC active layers

In order to investigate the role of the top interface (i.e. the interface between the active layer and MoO3 /Silver), photo-degradation tests in inert conditions are performed on samples that contain a stack of ITO/ZnO/Active layer only, refer to as incomplete OSCs. The incomplete OSCs are light-irradiated in inert atmosphere for 40 hours and then the fabrication is completed by the thermal

105

The effects of processing additives on the stability of OSCs

evaporation of MoO3 /Silver after that. The photovoltaic performances are measured and compared to those of fresh OSCs. The normalized PCEs are reported in Table 7.5. OSCs with ODT present a slight increase in Jsc , Voc and PCE. The increase in these parameters can be attributed to external parameters such as the increase in the work function of silver or internal parameters such as changes in the ZnO layer upon light irradiation. In this experiment, ODT-treated OSCs remain the most photo-stable. On the other hand, the photo-stability of OSCs with DPH, C-PYR and which are thermally annealed all exhibit similar performance loss: their PCEs decrease to around 80% of their initial values after 40 hours of light irradiation. This is in contrast with the trends observed in the photo-stability tests of complete OSCs shown earlier in sections 7.3.1 and 7.3.2 in which the stabilities were different. Table 7.5: Normalized photovoltaic performance of C-PYR, DPH and ODT-treated OSCs and thermally annealed OSCs subjected to light irradiation in inert atmosphere for 40 hours.

Normalized parameters [%] Types of OSCs

Jsc

Voc

FF

PCE

Rs

Rsh

Thermally annealed

86.3

93.3

88.6

82.8

164.4

111.1

C-PYR

93.4

95.7

92.8

83.0

202.1

48.3

DPH

100.8

92.3

83.6

78.5

333.6

58.2

ODT

109.0

101.1

94.5

104.4

153.0

71.3

The photovoltaic parameters of OSCs subjected to photo-degradation in the complete or incomplete way are compared to identify the role of the top electrode in photo-degradation. The normalized photovoltaic performances are compared after 40 hours of light irradiation (for the tests performed on complete OSCs, the electrical performances were determined by the extrapolation of the trends in Figure 7.9 at 40 hours). The normalized Voc and PCE in the two experiments are shown as a function of the processing conditions in Figure 7.16. 106

The effects of processing additives on the stability of OSCs 1 1 0

1 1 0

w ith o u t e le c tro d e w ith e le c tro d e

1 0 0

w ith o u t e le c tro d e w ith e le c tro d e

1 0 0

N o rm a liz e d P C E

N o rm a liz e d V o c

9 0

9 0

8 0

8 0 7 0 6 0

7 0 5 0 4 0

6 0

T A C

R -P Y

H D P

T A

T O D

(a)

R P Y C -

H D P

T O D

(b)

Figure 7.16: Comparison of the normalized (a) Voc and (b) PCE after 40 hours of light irradiation on OSCs without top electrode or with top electrode.

Interestingly, the OSCs subjected to light irradiation without top electrode appear to be more stable than the OSCs irradiated with the top electrode. In the photo-degradation tests on complete OSCs, the Voc is the parameter that discriminates the most the different types of processing conditions, whereas in the tests on incomplete OSCs, the Voc remains relatively similar for all the types of active layers. Two conclusions can be drawn: 1. The higher photo-stability of incomplete OSCs versus complete OSCs suggests that the presence of a top electrode plays a role in the photo-degradation. 2. The fact that the photo-stability of incomplete OSCs is less dependent on the processing conditions than in the case of complete OSCs suggests that the processing conditions affect the photo-stability at the top interface. Therefore, further investigations are performed on the photo-stability of the top interface.

7.6.2

Effects of light irradiation on the adhesion of the top electrode

Wang et al. demonstrated that light irradiation of an organic/metal stack caused deterioration at the interface by decreasing metal-organic bond density in organic light emitting diodes [220]. This photochemical deterioration at the interface caused a decrease in interfacial adhesion between the metal and the organic layer. 107

The effects of processing additives on the stability of OSCs

In order to investigate the degradation in OSCs at the top electrode, interfacial adhesion measurements are carried out on OSC stacks before and after light irradiation. The adhesion energy at the interface between the active layer and the top contact is measured using a four-point bending test configuration [14, 220]. During the adhesion experiment, a load is applied on the two sides of the sample under study. The load as a function of the displacement is recorded and a typical displacement plot is shown in Figure 7.17. The adhesion is the energy required

P/2

Pre - notch

P/2

Glass 2

Silver MoO3 Active Layer ZnO

Glass 1

(a)

(b)

Figure 7.17: (a) Schematic drawing of a four-point bend adhesion sample stack. A load P/2 is applied on each side of the sample. (b) A typical load versus displacement characteristic. Figure (b) is reproduced from reference [220] with permission of AIL Publishing LLC.

to cause delamination at the interface between two layers. Generally, the load first increases linearly with the displacement, which is characteristic for an elastic deformation. If a weak interface is present in the stack, a crack initiated by a pre-notch propagates along the interface. Such propagation causes a release of energy which can be related to the interfacial adhesion strength, Gc expressed as:

Gc =

21(1 − ν 2 )P 2 l2 16Eb2 h3

(7.1)

With ν the Poisson’s ratio of the glass substrate, E is the elastic modulus of the substrate, P the total force exerted onto the sample, b, h and l geometrical characteristics of the sample and the setup.

The OSC stack configuration is prepared on a rectangular glass substrate (1 cm x 5 cm) in order to be suitable for the four point bending test (Figure 7.17). 108

The effects of processing additives on the stability of OSCs

A second glass substrate is glued on top of the stack and a pre-notch is made on it. During the adhesion measurement test, equal loads are applied on both side of the sample. The loads deform the sample at a constant velocity of 0.25 µm.s−1 while the load as a function of the sample displacement is recorded. The adhesion measurement tests are performed on the two OSCs that exhibit the most differences in photo-stability: the thermally annealed and ODT-treated OSCs. Figure 7.18 shows the load versus displacement plots obtained. 6 4

5 ∆N

F o rc e [N ]

F o rc e [N ]

4

3

2 2

F re sh - O D T Irra d ia te d - O D T 1 6 .6 5

6 .7 0

6 .7 5

6 .8 0

3

F re s h - T h e rm a lly a n n e a le d Irra d ia te d - T h e rm a lly a n n e a le d

6 .8 5

6 .6 5

D is p la c e m e n t [m m ]

6 .7 0

6 .7 5

6 .8 0

6 .8 5

D is p la c e m e n t [m m ]

(a)

(b)

Figure 7.18: Load versus displacement characteristics for fresh and irradiated samples for (a) samples with ODT and (b) thermally annealed samples. On (a) the measured force loss (∆N) is showed. The plots of the irradiated samples were manually vertically down shifted for the clarity of the Figures.

The two samples demonstrate the same evolution: first, the load increases linearly with the displacement and when the displacement reaches 6 to 7 mm, a slight load drop is observed, followed by a linear increase in the load. Ultimately, for higher displacement values, the samples are broken. In case of complete delamination, a complete load drop would be observed (as depicted in Figure 7.17). In Figure 7.18, the plots show that no total delamination occurs. Instead, a small load drop is observed, suggesting that a crack propagates only slightly in the interface without fully delaminating it. At higher load values, the cracks continue to penetrate vertically throughout the stack and ultimately break the entire sample. As no total delamination occurs, the interface adhesion strength cannot be calculated using Equation 7.1. Instead, the load loss during the partial delamination (∆N) is determined (∆N is depicted in Figure 7.18). This load loss is related to the energy released during the partial delamination and therefore describes how far the delamination propagates at the interface. Table 7.6 and 109

The effects of processing additives on the stability of OSCs

Table 7.7 show the load loss values before and after degradation for thermally annealed and ODT-treated OSCs respectively. The values represent the average of two samples (except for the case of irradiated OSCs with ODT). Table 7.6: Force loss in the adhesion measurement test of thermally annealed samples.

∆N [N] Sample 1

Sample 2

Average

Fresh - TA

0.18

0.13

0.16

Irradiated - TA

0.33

0.23

0.28

76.8 %

Percentage increase

Table 7.7: Force loss in the adhesion measurement test of ODT-treated samples.

∆N [N] Sample 1

Sample 2

Average

Fresh - ODT

0.86

0.59

0.73

Irradiated - ODT

1.06

-

1.06a

46.0 %

Percentage increase

a The average takes into consideration the result of Sample 1 only.

110

The effects of processing additives on the stability of OSCs

Several observations and conclusions can be deduced form the results: 1. Regarding only fresh samples, the load loss in ODT-treated samples (0.73 N) is higher than the load loss in thermally annealed samples (0.16 N). This indicates that the interfacial adhesion is dependent on the processing conditions of the active layer. The AFM images of P3HT/PC61 BM-films treated with ODT and thermal annealed are recorded and depicted in Figure 7.19. The roughness measurement show that film treated with ODT presents a higher roughness than thermally annealed films. The difference in surface topography are likely the cause of the differences observed in the adhesion tests. 2. The load loss in photo-degraded OSCs is larger than that observed in fresh OSCs indicating that the interfacial delamination is more severe in photo-degraded OSCs. This, in turn, strongly suggests that the light irradiation affects the OSCs at the top interface by weakening the interfacial adhesion. 3. The percentage of load loss increase in thermally annealed samples is calculated to be 76.8%, whereas in ODT-samples, the increase is only 46.0%. This suggests that the interfacial adhesion is more severely deteriorated in the case of thermally annealed samples. Table 7.8: Roughness of active layers with different processing additives.

Roughness [nm]

Thermally annealed

1.6 vol% of ODT

1.4

7.9

Figure 7.19: AFM topography (1 µm x 1 µm) images of active layers films: (a) thermally annealed and (b) ODT-treated.

111

The effects of processing additives on the stability of OSCs

7.7

Conclusions

In this chapter, the effects of thermal annealing and the use of additive on the stability of OSCs are investigated. The main finding of this chapter is the OSC photo-stability dependence on the active layer processing conditions, a result which is valid for photo-degradation in both inert atmosphere as well as in air. The best photo-stability is obtained with ODT-treated OSCs, followed by CPYR and DPH-treated OSCs as intermediates, and thermally annealed OSCs as the least stable devices. The photo-degradation is shown to be unrelated to the remaining traces of additive in the active layer and to chemical degradation in the active layer. Instead, the photo-stability tests presented in Sections 7.5 and 7.6.1 suggest that the photo-degradation occurs primarily at the top interface. By means of adhesion measurements it is confirmed that light irradiation causes interfacial degradation which results in a weaker adhesion of the top electrode upon light irradiation. These results suggest that the increased in Rs observed in the photo-stability tests (in Sections 7.3.2 and 7.3.1) is due to increased contact resistance at the top electrode, which in turn decrease the Voc by affecting the built-in voltage in the device. Additionally, adhesion measurements show that ODT-treated OSCs are in a lower extend subjected to interfacial photo-degradation than thermally annealed OSCs. This is consistent with the fact that the photo-stability of ODTtreated OSCs is better than thermally annealed OSCs. Overall, processing additives are not detrimental for the photo-stability of OSCs compared to thermally annealed OSCs. In the specific case of ODT, the photostability is even improved. This shows that next to improving the efficiency, additives can also be employed to improve the photo-stability.

112

Chapter 8

Studies on new generation donor polymers

113

Studies on new generation donor polymers

As suggested in Chapter 2, the selection and the effects of processing additives depend on the type of D/A systems. In order to broaden the scope of the study on processing additive, other types of D semiconductors are investigated. P3HT can be classified as a low mobility semi-crystallline polymer. In this chapter, two other families of D are studied: a high mobility semi-crystalline polymer (a 1,4-diketopyrrolo[3,4-c]pyrrole derivative with a quaterthiophene substituent (PDQT)) and an amorphous polymer (poly[N-9’-heptadecanyl-2,7-carbazole-alt5,5-(4’,7’-di-2-thienyl-2’,1’,3’-benzothiadiazole (PCDTBT)).

8.1 8.1.1

Studies on PDQT Introduction to DPP based copolymers

In the past few years, 1,4-diketopyrrolo[3,4-c]pyrrole (DPP) has attracted considerable attention as an electron acceptor building block in conjugated copolymers for applications in OTFTs [118] and OSCs [79–81, 119–121]. The general chemical structure of DPP-based copolymers, depicted in Figure 8.1, contains an alternation of electron donating and DPP building blocks.

Figure 8.1: On the left: DPP-based conjugated polymers where R is a substituent, Donor 1 and Donor 2 are electron donating building blocks. On the right, examples of electron donating building blocks are depicted. Adapted from reference [121] with permission of the Royal Society of Chemistry .

The DPP-based copolymer investigated in this work contains a quaterthiophene group as the electron donor to form the PDQT copolymer depicted in Figure 8.2. This copolymer was shown to form films with a high degree of crystallinity due to strong intermolecular π-π stacking. As a result, PDQT was shown to exhibit high hole mobility in OTFT configuration with mobilities up to 0.97 cm2 .V−1 .s−1 [122, 123]. Another interesting feature of PDQT is its large absorption spectrum which 114

Studies on new generation donor polymers

is extended to the near IR (∼ 950 nm), as depicted in the UV-Vis absorption spectrum of PDQT/PC61 BM film in Figure 8.3.

Figure 8.2: Chemical structure of PDQT.

A b s o r b a n c e [ a .u ]

0 .6

0 .4

0 .2

0 .0 4 0 0

5 0 0

6 0 0

7 0 0

8 0 0

9 0 0

1 0 0 0

W a v e le n g th [n m ]

Figure 8.3: Solid-state UV-Vis spectrum of PDQT/PC61 BM in a 1/3 ratio.

The HOMO LUMO energy levels of PDQT are estimated to be respectively and 5.3 eV and 4.0 eV respectively [152]. Such a LUMO energy level leads to an energy difference of 0.3 eV with the LUMO of PC61 BM and is therefore suitable for efficient charge transfer. The energy diagram of PDQT/PC61 BM-based OSC is depicted in Figure 8.4.

8.1.2

Performance of OSCs based on PDQT/PC61 BM

Prior to the fabrication of OSCs, the HSPs of PDQT are determined in order to verify its solubility in conventional solvents such as CB or ODCB.

115

Studies on new generation donor polymers PC60BM PDQT

ZnO

ITO

Ag

MoO3 2.3 eV

4.0 eV

4.3 eV

4.4 eV

4.7 eV

4.7eV 5.3 eV

5.3 eV

6.1 eV 7.7 eV

Figure 8.4: Energy levels of PDQT/PC61 BM - OSC in an inverted architecture.

The HSPs of PDQT have been determined experimentally by performing solubility tests. The HSPs resulting from the fitting are presented in Table 8.1 and the graphical representation of the PDQT solubility sphere is depicted in Figure 8.5. Table 8.1: HSPs of PDQT. δD [MPa1/2 ] δP [MPa1/2 ] δH [MPa1/2 ] PDQT

18.28

4.11

RO [MPa1/2 ]

FIT

3.2

0.860

3.07

PCBM 20 15

δ H [√MPa]

PDQT 10 5 0 30

−5 −10 15

20 10

δ D [√MPa]

5

0

δ P [√MPa]

−5 10

Figure 8.5: Solubility spheres of PC61 BM and PDQT.

Using these HSPs, the RED values between PDQT - CB and PDQT - ODCB are calculated and found to be respectively 0.56 and 0.90. These numbers suggest that PDQT is soluble in both solvents. It was shown in the literature, that PDQT-based OSCs can be fabricated with thick active layers (up to ∼ 800 116

Studies on new generation donor polymers

nm) [151]. As such active layers are usually obtained from low boiling point solvents, CB is chosen as the host solvent due to its low boiling point (51 °C below that of ODCB) in order to fabricate thick active layer OSC. PDQT and PC61 BM formulations are prepared with a total solid content of 30 mg.mL−1 in CB with various D/A ratio (from 1/1 to 1/4) and spin-cast at a spin speed of 800 rpm to form the active layer. Table 8.2 shows the photovoltaic performance of inverted OSCs prepared from various D/A ratios. Table 8.2: Photovoltaic parameters of OSCs with various ratios of PDQT/PC61 BM. D/A ratio

Jsc [mA.cm−2 ]

Voc

FF

[V]

PCE

Rs

Rsh

[%]

[Ohm.cm2 ]

[Ohm.cm2 ]

1/4

2.6

±0.1

0.58

±0.01

0.51

±0.01

0.8

±0.0

14.5

±1.9

1607

±263

1/3

3.3

±0.1

0.60

±0.00

0.57

±0.03

1.1

±0.1

10.1

±1.7

1698

±369

1/2

2.8

±0.1

0.59

±0.01

0.51

±0.03

0.8

±0.0

14.3

±2.5

1245

1/1

4.8

±1.4

0.60

±0.00

0.42

±0.07

1.2

±0.2

13.6

±0.4

501

3/1

0.8

±0.1

0.59

±0.02

0.50

±0.07

0.2

±0.0

±33.7

2412

78.8

±99

±419 ±624

OSCs with a D/A ratio of 3/1 exhibit a low PCE of 0.24%, mostly due to low Jsc . This low Jsc is likely to be due to charge transport issues also evident from the high Rs . Overall, the PCEs of OSCs are similar for each D/A ratio, with the exception of the 3/1 one. XRD measurements are conducted on PDQT/PC61 BM - films. The XRD patterns are presented in Figure 8.6. Pure PDQT exhibits a diffraction peak at 4.47° which corresponds to the interlayer spacing d(100) between PDQT chains, suggesting a predominant edge-on orientation of the chains. Table 8.3 presents the angles of the maximum diffraction peak and the crystallite domain sizes estimated with Scherrer’s equation for each of the films. For each film, the crystallite domain size is 13 - 14 nm. This result suggests that there is no impact of the proportion of A on the crystallization of PDQT during film formation. The crystalline structure of PDQT can be observed in the AFM images depicted in Figure 8.7. The phase image of pure PDQT exhibits well defined crystallites that appear to be homogeneous in size and fairly flat as suggested by its low roughness of 1.8 nm. In PDQT/PC61 BM-films, the phase images show large features, heterogeneous in size, suggesting the coexistence of PDQT crystallites 117

Studies on new generation donor polymers 2 5 0 0 0

4 0 0 0 0

P D Q T P D Q T ra tio :

3 5 0 0 0

1 /1 1 /2 1 /3 1 /4

3 0 0 0 0

C o u n ts ( a .u .)

C o u n ts ( a .u .)

2 0 0 0 0

1 5 0 0 0

2 5 0 0 0 2 0 0 0 0 1 5 0 0 0

1 0 0 0 0 2

3

4

5

6

1 0 0 0 0 7

2

3

4

2 θ (°)

5

6

7

2 θ (°)

(a)

(b)

Figure 8.6: XRD patterns of: (a) pure PDQT and (b) PDQT/PC61 BM blends in various D/A ratios. Table 8.3: Diffraction peaks and domain sizes of PDQT/PC61 BM with various D/A ratios. D/A system

2θ [°]

Domain sizes [nm]

Pure PDQT

4.47

14

Ratio 1/1

4.45

13

Ratio 1/2

4.45

14

Ratio 1/3

4.53

13

Ratio 1/4

4.39

14

and PC61 BM crystallites. The increase in PC61 BM proportion also leads to a gradual increase in roughness: the roughness increases from 1.8 nm for pure PDQT to 4.0 nm in blends of PDQT/PC61 BM prepared in a 1/4 ratio. The fact that the films present such a heterogeneous distribution of crystallite sizes suggests that PDQT and PC61 BM aggregate independently, explaining the non dependence of PDQT crystallite sizes on the proportion of A.

8.1.3

Effects of processing additives

Following the same approach as the one applied to the P3HT/PC61 BM system, processing additives are selected under the conditions that they are good solvents for PC61 BM and poor solvents for PDQT. Using the HSPs as a numerical tool to determine such solvents, ODT was found to have RED values with PDQT and PC61 BM of respectively 1.32 and 0.68. Therefore, ODT is investigated as 118

Studies on new generation donor polymers 1.35 V

400nm

2.67 V

400nm

(a)

400nm

(d)

0.00 V 30.19 nm

400nm

(f)

0.00 nm

400nm

(c)

0.00 V 23.16 nm

400nm

(e)

400nm

(b)

0.00 V 15.96 nm

2.55 V

1.84 V

0.00 V 26.64 nm

400nm

(g)

0.00 nm

(h)

0.00 nm

0.00 nm

Figure 8.7: AFM images of PDQT/ PC61 BM in various D/A ratios: (a-d) phase images, (e-h) topography images: (a) and (e) ratio pure PDQT, (b) and (f) ratio 2/1, (c) and (g) ratio 1/2, (d) and (h) ratio 1/4.

a processing additive in PDQT/PC61 BM - OSCs. DIO is also studied as a processing additive because several studies reported on its success in increasing the efficiency of DPP-based copolymer OSCs [120, 195]. The solubility properties of DIO are similar to the ones of ODT in the sense that PDQT has a lower solubility in DIO than in halogenated solvents [195]. OSCs are fabricated utilizing formulations that contain various concentrations of ODT or DIO. The formulations are prepared from a D/A ratio of 1/3 and a total solid content of 30 mg.mL−1 . Figure 8.8 shows the photovoltaic parameters (Voc , Jsc , FF and PCE) of the OSCs with the different processing additives and their corresponding concentrations. The best values for each system are recorded in Table 8.4. Table 8.4: Electrical performance of PDQT/PC61 BM-OSCs with and without additive. Type of

Jsc

Voc

additive

[mA.cm−2 ]

[V]

FF

PCE

Rs

Rsh

[%]

[Ohm.cm2 ]

[Ohm.cm2 ]

No additive

4.2

± 0.3

0.58

± 0.00

0.62

± 0.00

1.5

± 0.1

12.2

± 0.3

905

± 43

ODT - 5 vol%

5.8

± 0.1

0.62

± 0.00

0.52

± 0.01

1.9

± 0.1

10.1

± 0.5

604

± 49

DIO - 5 vol%

6.2

± 0.2

0.58

± 0.00

0.60

± 0.02

2.1

± 0.0

11.7

± 0.3

1040

119

± 241

Studies on new generation donor polymers

5 vol% of ODT and DIO increase the PCE by respectively 26% and 46% in comparison to control devices devoid of processing additives, resulting in a PCE of 1.85% in case of ODT and 2.14% in the case of DIO. The main parameter responsible for the increase in PCE is the Jsc , similarly to the case of P3HT/PC61 BM - OSCs. The Jsc increases with additive concentrations up to 5 vol% but decreases at higher concentrations. In contrast to the case of P3HT/PC61 BM, no significant change in Voc is observed (except for OSCs fabricated with 11 vol% of ODT). This suggests that processing additives do not lead to significant change in the overall crystallinity, certainly because PDQT is already highly crystalline in PDQT/PC61 BM-films devoid of processing additives. 6 .0

(b )

(a ) 0 .6

5 .5

[V ]

[ m A .c m

-2

]

5 .0

0 .4

4 .0

J

S C

V

O C

4 .5

O D T D IO

3 .5

0 .2

3 .0 0

2

4

6

8

1 0

0

1 2

0 .7

2

4

6

8

1 0

1 2

1 0

1 2

C o n c e n tra tio n o f a d d itiv e [v o l% ]

C o n c e n tra tio n o f a d d itiv e [v o l% ]

(c )

(d ) 2 .0

0 .6 1 .5

P C E [% ]

F F

0 .5

0 .4

1 .0

0 .3 0 .5 0 .2 0 .0 0

2

4

6

8

1 0

1 2

0

C o n c e n tra tio n o f a d d itiv e [v o l% ]

2

4

6

8

C o n c e n tra tio n o f a d d itiv e [v o l% ]

Figure 8.8: Electrical performance of PDQT/PC61 BM-OSCs as a function of additive concentration: (a) Jsc , (b) Voc , (c) FF and (d) PCE.

XRD measurements are conducted on PDQT/PC61 BM-films that contain various concentrations of DIO. The measurement of the crystallite sizes using the Scherrer equation shows that the sizes of PDQT crystallites increase upon the 120

Studies on new generation donor polymers

introduction of DIO. Films prepared from 5 vol% and 11 vol% of DIO exhibit crystallite sizes of 19 and 21 nm respectively while PDQT/PC61 BM-films devoid of processing additive exhibit a crystallite size of 14 nm. The appearance of large crystallite sizes are in agreement with the features observed in the AFM images of PDQT/PC61 BM films depicted in Figure 8.9. Films prepared from 11 vol% of DIO reveal large features which can be attributed to large PDQT crystallites. 17.19 nm

400nm

400nm

(a)

(c) 0.00 nm

0.00 nm

54.91.83 degV

34.81.16 degV

903.05 mV 1.83 V 27.09 deg

400nm

400nm

(d)

400nm

(b)

0.00 nm

400nm

25.13 nm

17.72 nm

(e)

0.00mV V 00.00 deg

0.00 V

0 deg

(f)

Figure 8.9: AFM images of PDQT/PC61 BM in a 1/3 ratio prepared with various concentrations of additive: (a-c) topography images, (d-f) phase images: (a) and (d) no additive (b) and (e) 5 vol%, (c) and (f) 11 vol%.

121

0.00 V

0 deg

Studies on new generation donor polymers

8.1.4

Discussions and conclusion

The introduction of ODT and DIO successfully increases the PCE of PDQT/PC61 BM - based OSCs. In this system, the selection rules identified previously for P3HT/PC61 BM OSCs apply. The additive is a poor solvent for the D and a better solvent for the A. However, the role of the additive in the BHJ morphology depends on the type of D used. In the case of P3HT/PC61 BM, processing additives decrease the average crystallite domain size whereas they increase it in the case of PDQT/PC61 BM. The origins of the morphological differences between PDQT and P3HT are discussed here. The AFM images and the XRD measurements show that the molecular arrangement of PDQT chains are significantly different from that of P3HT. When blended with PC61 BM, PDQT forms large domains while untreated P3HT is known for intermixing with PC61 BM [86]. Clearly, PDQT has a stronger tendency to crystallize than P3HT. The morphological differences can naturally be attributed to the different interactions caused by the difference in polymer backbone. However, the morphology is not dependent solely on the polymer backbone, more general parameters also need consideration. Regarding the molecular weight, several studies in the literature revealed that this parameter significantly affects the aggregation of polymer chains and thus the photovoltaic performance [75, 79, 99]. For example, Kline et al. showed that low molecular weight P3HT chains are more ordered than high molecular ones which substantially affects the hole mobility [99]. He et al. showed that the photovoltaic performance and the film morphology of a series of PTB7-based polymers significantly vary with the molecular weight. In their study, high molecular weight (above 150 kg/mol) appears to be necessary for attaining high efficiency [75]. It is noteworthy to point out that the molecular weight of the PDQT used here is much lower than that of P3HT: 21,200 Da for PDQT and 53,000 Da for P3HT. Such large difference in molecular weight can be expected to contribute in explaining the difference in morphological behaviour. Next, the chemical structure of the polymer side chains also needs consideration. Generally, long side chains are desirable for increasing the solubility of polymers but besides their effects on the solubility, the side chains were also shown to affect the domain sizes of polymer in films. A study from Li et al. on DPP-based 122

Studies on new generation donor polymers

polymers showed that increasing side chains length increases the diameter of the D fibrils which was detrimental for the performance. The efficiency of OSCs was best for polymer with small side chain lengths [120]. PDQT possesses large substituents (Figure 8.2) wich are consequently also expected to contribute in its aggregation behavior. Further investigation is required to identify and to understand the influence of the polymer backbone and the more general polymer properties such as the molecular weight and the side chain length on the morphology. The effects of additives on the morphology of diverse D/A system needs to consider the above parameters.

8.2 8.2.1

Studies on PCDTBT Introduction to carbazole based copolymers

Poly(2,7-carbazole) derivatives represent an efficient family of copolymers for BHJ-OSCs [18, 19]. The carbazole is an electron rich moiety that consists of two six-membered benzene rings fused on either side of a five-membered nitrogencontaining ring (See Figure 8.10a).

(a)

(b)

Figure 8.10: Chemical structures of: (a) a carbazole unit and (B) PCDTBT.

Among all the derivatives, PCDTBT represents a good candidate as a D material in OSCs [19, 46, 166, 201, 215]. Its chemical structure is presented in Figure 8.10b. One of the interesting features of PCDTBT is the low lying HOMO level (5.5 eV) which is suitable to obtain OSCs with high Voc [166, 213]. Using Scharber’s equation for the estimation of Voc (Equation 1.5) [185], a Voc of 0.9 V is expected. The energy diagram of PCDTBT/PC61 BM-based OSC is depicted in Figure 8.11. 123

Studies on new generation donor polymers ITO

ZnO

PC60BM PCDTBT MoO3

Ag

2.3 eV

4.4 eV

4.3 eV

3.6 eV 4.7 eV

4.7eV 5.5 eV

5.3 eV

6.1 eV 7.7 eV

Figure 8.11: Energy levels of PCDTBT/PC61 BM-OSCs in an inverted architecture.

Unlike P3HT and PDQT, PCDTBT is an amorphous polymer which was reported to highly mix with PC61 BM resulting in poor D/A phase separation [36, 221]. Such morphology prevents PC61 BM to form A domains with sizes that are efficient for charge transport to the electrode. As a result, the efficiency of OSCs is limited by the charge transport. In order to overcome these issues, the active layer generally needs to fulfill two requirements. First, PCDTBT/PC61 BM active layers are required to be prepared with a high proportion of A in order to exhibit larger A domains. In the literature, efficiencies of 6% to 7% were obtained from formulations containing a D/A ratio between 1/2 and 1/4 [129, 146, 201]. The other requirement for high efficiency PCDTBT based OSCs is a thin active layer, generally ∼ 70 - 100 nm [16]. Thicker films exhibit inefficient charge collection and therefore low FF and Jsc [146]. However, the D semiconductor is the main absorbing material. Therefore, such active layers (thin and with a high proportion of A) optimize charge collection at the expense of light absorption. Optimization of the morphology can help improving charge collection without sacrificing light absorption. To this end, an investigation on formulation strategies is carried out.

8.2.2

Solubility properties of PCDTBT

The HSPs of PCDTBT are determined experimentally by performing solubility tests. The HSP values resulting from the fitting are presented in Table 8.5 and the graphical representation of the PCDTBT solubility sphere is depicted in Figure 8.12. In Figure 8.12, the solubility sphere of PCDTBT visually appears to be relatively close to the one of PC61 BM suggesting a certain affinity between them. The 124

Studies on new generation donor polymers Table 8.5: HSPs of PCDTBT. δD

δP

δH

RO

FIT

[MPa1/2 ] [MPa1/2 ] [MPa1/2 ] [MPa1/2 ] PCDTBT

19.10

3.50

5.19

5.90

RED (with PC61 BM)

0.98

0.30

Figure 8.12: Solubility spheres of PCDTBT (in blue) and PC61 BM (in red) in the Hansen solubility space.

RED between PCDTBT and PC61 BM can be calculated in order to estimate their affinity. Graham et al. used this method to predict the extent of phase separation between small molecules [63]. Here, the RED between PCDTBT and PC61 BM is calculated to be 0.30 MPa1/2 . For comparison, PC61 BM has a RED of 0.50 MPa1/2 with P3HT and a RED of 0.55 MPa1/2 with PDQT. These results suggest that PC61 BM has a higher affinity with PCDTBT than with P3HT or PDQT, which is in line with the highly mixed interpenetrated network formed between PCDTBT and PC61 BM.

125

Studies on new generation donor polymers

8.2.3

Performance of OSCs based on PCDTBT/PC61 BM.

Preliminary optimizations on PCDTBT/PC61 BM - based OSCs are carried out to define the optimum D/A ratios and film thicknesses. Table 8.6 lists the D/A ratios, the solid contents and the resulting thicknesses investigated. Table 8.6: Properties of PCDTBT/PC61 BM active layers investigated. D/A ratio 1/1

1/2

Solid content

Spin-casting speed

Thickness

1500 rpm

159 nm

1000 rpm

322 nm

1500 rpm

232 nm

2000 rpm

170 nm

1600rpm

108 nm

800 rpm

266 nm

1000 rpm

230 nm

1500 rpm

177 nm

2000 rpm

137 nm

700 rpm

113 nm

1300 rpm

91 nm

1900 rpm

69 nm

700 rpm

83 nm

1300 rpm

51 nm

1900 rpm

44 nm

-1

15 mg.mL

25 mg.mL-1

-1

20 mg.mL

-1

25 mg.mL

1/3

-1

20 mg.mL

1/4

20 mg.mL-1

Figure 8.13 shows the electrical performance (Voc , FF, Jsc and PCE) of OSCs prepared with the parameters listed in Table 8.6. Overall, the results suggest that the electrical parameters are primarily dependent on the thickness. PCEs over 3% can be obtained for active layer thicknesses between 90 nm and 130 nm. The highest PCE of 4.44% is obtained for OSCs with a D/A ratio of 1/2 in an active layer of 108 nm. DIO and ODT are investigated as processing additives for the PCDTBT/PC61 BM system. In contrast to PDQT and P3HT, the HSPs show that DIO and ODT are located inside the solubility spheres of PCDTBT and PC61 BM as depicted in Figure 8.14.

126

Studies on new generation donor polymers 1 4

r a tio r a tio r a tio r a tio

1 2

1 :1 1 :2 1 :3 1 :4

1 .4

r a tio r a tio r a tio r a tio

1 .2

1 0

1 :1 1 :2 1 :3 1 :4

V o c [V ]

Jsc [mA/cm²]

1 .0

8 6

0 .8 0 .6 0 .4

4

0 .2

2

5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

5 0

1 0 0

1 5 0

T h ic k n e s s [n m ]

2 0 0

2 5 0

3 0 0

3 5 0

T h ic k n e s s [n m ]

(a)

(b) 6

0 .7 0

r a tio r a tio r a tio r a tio

0 .6 5 0 .6 0 0 .5 5

r a tio r a tio r a tio r a tio

1 :1 1 :2 1 :3 1 :4

5

1 :1 1 :2 1 :3 1 :4

4

P C E [% ]

0 .5 0

F F

0 .4 5 0 .4 0

3 2

0 .3 5 0 .3 0

1

0 .2 5 0 .2 0

0 5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

5 0

1 0 0

1 5 0

2 0 0

2 5 0

3 0 0

3 5 0

T h ic k n e s s [n m ]

T h ic k n e s s [n m ]

(c)

(d)

Figure 8.13: Photovoltaic parameters of OSCs as a function of the thickness for various D/A ratios: (a) Jsc , (b) Voc , (c) FF and (d) PCE.

15

δ H [√MPa]

10

5 30

0

−5 15

20 10

δ D [√MPa] 5

0

δ P [√MPa]

−5 10

Figure 8.14: Hansen solubility spheres of PCDTBT and PC61 BM and the following solvents: DIO (black), ODT (yellow).

In Figure 8.15, the UV-visible absorption spectra of films prepared with and without DIO show that the introduction of DIO leads to a red-shift of PCDTBT absorption peak. The wavelength of PCDTBT absorption maximum is 568 nm 127

Studies on new generation donor polymers

in films without DIO and increases to 582 nm in films prepared with 5 vol% of DIO. Similarly, ODT causes a red-shift of the absorption peak (Table 8.7).

A b s o r b a n c e [ u .a ]

0 .6

0 v o l% 1 v o l% 5 v o l%

D IO D IO D IO

0 .4

0 .2

0 .0

3 0 0

4 0 0

5 0 0

6 0 0

7 0 0

8 0 0

W a v e le n g th [n m ]

Figure 8.15: Solid-state UV-Vis absorption spectra of PCDTBT/PC61 BM films (ratio 1/3) prepared without or with DIO. Table 8.7: Absorption maximum of PCDTBT/PC61 BM films with or without processing additive. Control 0 Absorption maximum [nm]

568

DIO 1 vol% 5 vol% 576

582

ODT 1 vol% 5 vol% 572

578

This red-shift is similar to the effects of DIO or ODT on other D polymers and is attributed to an increased degree of π-conjugation [5, 66, 89]. The UV-Vis absorption spectra therefore suggest that DIO and ODT are effective in increasing the degree of ordering within PCDTBT chains. The effects of DIO and ODT on the photovoltaic performance of OSCs are investigated by preparing OSCs from solutions containing DIO or ODT in various concentrations (from 0.5 vol% to 2 vol%). The results show that the introduction of these solvents decreases the performance of OSCs. This suggests that increased order within the polymer chains does not improve the photovoltaic performance of PCDTBT/PC61 BM - based OSCs. Next to formulations with DIO and ODT, several other solvent formulations are tested.

OSCs are fabricated using the widely used 1-chloronaphthalene

as a processing additive, xylene as a good solvent for both semiconductors, NMP as a good solvent for PC61 BM and poor solvent for PCDTBT. None of 128

Studies on new generation donor polymers

these formulations were successful in improving the photovoltaic performance of PCDTBT/PC61 BM - OSCs. It is now noteworthy to point out that all these approaches are generally used to enhance the aggregation or the crystallinity of D polymers. The fact that all of these approaches are unsuccessful suggests that increasing the ordering of PCDTBT chains is inefficient in increasing the PCE of PCDTBT/PC61 BM - OSCs. Next, studies on the aggregation of PC61 BM are carried out.

8.2.4

PCDTBT/PC61 BM/C60 ternary blend.

This next study focuses on the effect of C60 as a nucleating agent for PC61 BM in the PCDTBT/PC61 BM system. Because of the low solubility of C60 in ODCB [90, 189], C60 is expected to undergo the transition from the liquid state to the solid state at an earlier stage than PC61 BM during the process of film formation. The introduction of C60 is therefore expected to alter the aggregation of PC61 BM. Systems based on a ternary blend of PC61 BM and C60 as the acceptor components, and PCDTBT as the donor are studied. The resulting active layers are of the type: PCDTBT/PC61 BM(1−x) /C60(x) with x the fraction of C60 varying from 0 to 1. The UV-Vis spectra of ternary blends with C60 fractions of 0, 0.4 and 0.7 are depicted in Figure 8.16. The UV-Vis spectra show that the introduction of C60 causes a blue-shift in the absorption peak of PCDTBT. The wavelength of PCDTBT absorption maximum is 564 nm in films without C60 and decreases to 558 nm in PCDTBT/PC61 BM(0.3) /C60(0.7) films. In contrast to the red-shift caused by increased ordering within polymer chains, the blue-shift can be interpreted as a reduction of polymer chain ordering. Next, the electron mobility in a ternary blend is measured and compared to the electron mobilities of PCDTBT/PC61 BM in various D/A ratios. The electron mobilities are measured in OTFT configurations for PCDTBT/PC61 BM systems with D/A ratios of 1/4, 1/3 and 1/2 and for the ternary blend PCDTBT/PC61 BM(0.6) /C60(0.4) at a D/A ratio of 1/2. Figure 8.17a depicts the transfer characteristics of the resulting n-type OTFTs. The OTFT mobilities are calculated at different values of gate voltage and Figure 8.17b depicts the mobility as a function of VGS -Vth . 129

Studies on new generation donor polymers

F ra c tio n o f C

6 0

:

0 0 .4 0 .7

A b s o r b a n c e [ a .u .]

1 .0

0 .5

0 .0

3 5 0

4 0 0

4 5 0

5 0 0

5 5 0

6 0 0

6 5 0

7 0 0

W a v e le n g th [n m ]

Figure 8.16: Solid state UV-Vis spectra of PCDTBT/PC61 BM(1−x) /C60(x) films with various fraction of C60 .

Table 8.8 displays the average electron mobility measured from 2 to 4 OTFTs at VGS -Vth = 4 V. 8 0

R a tio 1 /4 R a tio 1 /3 R a tio 1 /2 R a tio 1 /2 w ith a C 6 0 f ra c tio n o f 0 .4

0 .0 0 5 -1

6 0

R a tio R a tio R a tio R a tio

0 .0 0 6

.s ]

7 0

0 .0 0 4

.V 2

4 0

M o b ility [c m

Id s (n A )

6 0

fr a c tio n o f 0 .4

-1

5 0

1 /2 1 /3 1 /4 1 /2 w ith C

3 0 2 0 1 0

0 .0 0 3 0 .0 0 2 0 .0 0 1 0 .0 0 0

0 0

5

1 0

1 5

V

G S

2 0

2 5

3 0

-2

0

(V )

2

4

V g -V th [V ]

(a)

(b)

Figure 8.17: (a) Transfer characteristics and (b) Mobility as a function of VGS -Vth for various D/A ratios and for PCDTBT/PC61 BM(0.6) /C60(0.4) .

Regarding the effects of processing additives on the electron mobility, the results show that increasing proportion of A in the D/A blend increases the mobility. This result is expected since a high proportion of A leads to an increased amount of A pathways. For a D/A ratio of 1/2, the electron mobility increases significantly with the introduction of C60 : the ternary blend exhibits an electron mobility six times higher than that of a blend without C60 (Table 8.8). Such an increase in electron mobility shows that the introduction of C60 has an effect on the aggregation of PC61 BM. Interestingly, the mobility of the ternary blend in a 1/2 ratio is also much higher than that of PCDTBT/PC61 BM in a 1/4 ratio 130

Studies on new generation donor polymers Table 8.8: Average electron mobility measured in a saturation regime at VGS -Vth = 4 V. Electron mobility

Formulations

x10-3 [cm2 .V-1 .s-1 ] Ratio 1/4

1.56 ± 0.29

Ratio 1/3

1.13 ±0.09

Ratio 1/2

0.48 ±0.08

Ratio 1/2 in the ternary blend

4.96 ±0.45

(which contains more PC61 BM). This suggests that the introduction of C60 results in a particular electron mobility that cannot be obtained in binary blend, even when fabricated with a high proportion of A. Series of OSCs are prepared from ternary blends: PCDTBT/PC61 BM(1−x) /C60(x) , with a D/A ratio kept constant and x the fraction of C60 varied from 0 to 1. The energy levels of such ternary-blend OSCs are displayed in Figure 8.18. Two D/A ratios are studied: 1/3 and 1/2. ITO

ZnO

PC60BM PCDTBT MoO3 2.3 eV

C60 3.9 eV

4.4 eV

4.3 eV

3.6 eV 4.7 eV

4.7eV 5.8 eV

7.7 eV

Ag

5.5 eV

5.3 eV

6.1 eV

Figure 8.18: Energy levels of PCDTBT/PC61 BM/C60 - OSCs in an inverted architecture.

Figure 8.19 shows the photovoltaic parameters (Voc , Jsc , FF and PCE) of ternary OSCs as a function of the fraction of C60 . The Jsc and the FF are not severely impacted by the introduction of C60 , with the exception of the OSC prepared from only C60 at a 1/2 ratio. The PCE remains relatively unchanged for a C60 fraction ≤ 0.7 and drops significantly when C60 is the only A component. The poor photovoltaic performance obtained in the case of PCDTBT/C60 -OSCs can be due to the low solubility of C60 in ODCB (23 to 27 131

Studies on new generation donor polymers 1 1

1 .0

( (

R a tio 1 /2 ) R a tio 1 /3 )

1 0

Jsc [mA/cm²]

V o c [V ]

0 .9

0 .8

0 .7

9

8

7 0 .6

(a )

(b ) 6

0 .0

0 .2

0 .4

0 .6

0 .8

0 .0

1 .0

0 .2

0 .4

0 .6

0 .8

1 .0

0 .8

1 .0

F ra c tio n o f C 6 0

F ra c tio n o f C 6 0 5 .5

0 .6 0

5 .0 0 .5 5

4 .5 0 .5 0

P C E [% ]

4 .0

F F

0 .4 5

0 .4 0

3 .5 3 .0 2 .5

0 .3 5

(c )

2 .0

(d )

1 .5

0 .3 0 0 .0

0 .2

0 .4

0 .6

0 .8

0 .0

1 .0

0 .2

0 .4

0 .6

F ra c tio n o f C 6 0

F ra c tio n o f C 6 0

Figure 8.19: Electrical parameters of OSCs with various fraction of C60 : (a) Voc , (b) Jsc , (c) FF and (d) PCE.

mg.mL−1 [90, 189]) which is likely to cause coarse aggregates. The Voc is highest in OSCs with only PC61 BM as the A component (0.90 V for the 1/2 ratio and 0.91 V for the 1/3 ratio) and decreases with increasing fraction of C60 . At a fraction x of 1, the Voc values drop to 0.62 V and 0.79 V for OSCs with D/A ratios of 1/2 and 1/3 respectively. The variation in Voc with the composition of the ternary blends has been the subject of investigation in the literature and remains under debate. Street et al. argued that a ternary blend can be described with an alloy model of D and A with a Voc depending on the average HOMO LUMO energy levels [198]. Such model fails to describe the present ternary blend because it predicts an increase in Voc with the introduction of C60 due to its higher lying LUMO level. As previously introduced in Section 1.4.4 (General factors influencing BHJ-OSC 132

Studies on new generation donor polymers

efficiency), presence of fullerene nanocrystals can shift the energy of the charge transfer state. Such shift is reported to decrease the Voc of OSCs [175]. Therefore, the decrease in Voc observed in this experiment suggest that the introduction of C60 increases the amount of fullerene nanocrystals (PC61 BM or C60 or a mixture of both). The increase in fullerene aggregation caused by the introduction of C60 is consistent with the trend in electron mobility observed from the OTFT mobility measurements, along with the disruption of polymer chain ordering suggested by the blue-shift in the UV-Vis absorption spectra. Such morphological changes caused by C60 do not affect the PCE. The effects of C60 introduction on OSC-stability is then studied. As presented in the literature section on OSC stability in chapter 7-section 7.1, PC61 BM is subject to severe thermal degradation. Upon prolonged thermal annealing, PC61 BM molecules can form micrometer size aggregates which are profoundly detrimental for the performance of OSCs. In PCDTBT/PC61 BM films, Derue et al. showed that thermal annealing above 160 °C causes the formation of micrometer sizes PC61 BM [56]. In the literature, several studies were carried out to improve the thermal stability of PC61 BM [222]. Among these, few studies reported on the beneficial effect of the introduction of C60 on the thermal stability of D/A blends based on P3HT/PC61 BM [178], or poly[2,3-bis-(3-octyloxyphenyl)quinoxaline5,8-diyl-alt thiophene-2,5diyl](TQ1)/PC61 BM [126]. For the study on OSC thermal stability, three types of ternary blend OSCs are fabricated with a D/A ratio kept constant at 1/2 and with a C60 fraction, x, of 0, 0.4 and 0.7. The ZnO layer and the active layer are deposited following the procedure used for the fabrication of inverted OSCs. After the deposition of the active layer, the samples are thermally annealed on a hot plate set at 160 °C for two or four hours. The top electrodes (MoO3 /Ag) are evaporated on top

of the thermally annealed samples. The J -V characteristics of these OSCs are measured and the photovoltaic performances are normalized with respect to the parameters of fresh OSCs. Figure 8.20 shows the evolution of normalized PCE as a function of the annealing time. The normalized photovoltaic performances of the OSCs after two hours of thermal treatment are listed in Table 8.9. After two hours of thermal treatment, the PCE of the binary blend PCDTBT/PC61 BM drops to 51.2% of the initial value. Ternary blend OSCs demonstrate a superior thermal stability with PCEs maintained at 83.0% and 94.2% of the initial PCEs 133

Studies on new generation donor polymers Table 8.9: Normalized photovoltaic parameters of ternary blend OSCs after two hours of thermal treatment at 160 °C. C60

Normalized parameters [%]

fraction

Jsc

Voc

FF

PCE

0

76.9

89.3

74.5

51.2

0.4

101.1

92.3

88.9

83.0

0.7

112.9

94.7

88.1

94.2

1 0 0

N o rm a liz e d P C E [% ]

9 0 8 0 7 0

Fraction of C60 :

6 0

0 0.4 0.7

5 0 4 0 0

2

4

Duration of thermal annealing at 160°C [hours] Figure 8.20: Normalized PCE as a function of the time of annealing treatment at 160o C.

for C60 fractions of 0.4 and 0.7 respectively. The thermal treatment decreases primarily the FF and Jsc of binary OSCs likely caused by the aggregation of PC61 BM. The ternary blends exhibit a different trend characterized by less decrease in FF and an increase in Jsc . This increase in Jsc is unlikely caused by morphological changes but can be attributed to other phenomenon such as the removal of residual solvent in the active layer upon thermal annealing. Overall, ternary blends demonstrate high thermal stability compared to the binary blend. This suggests that thermal annealing does not deteriorate the active layer morphology as severely as in the binary system. Interestingly, the thermal stability is dependent on the fraction of C60 : superior thermal stability is obtained for the ternary blend with a C60 fraction of 0.7 compared to 0.4. Microscopic images of PCDTBT/PC61 BM and ternary blend films are analyzed 134

Studies on new generation donor polymers

before and after thermal treatment of 2, 4 and 8 hours at 160 °C. The images are depicted in Figure 8.21. Figure 8.22 displays images with higher magnifications of blends with C60 fraction of 0 and 0.4. As shown in Figure 8.21, thermally treated PCDTBT/PC61 BMfilms demonstrate PC61 BM crystallites throughout the active layer. On the other hand, films prepared from ternary blends exhibit a surface without the presence of aggregates even after 8 hours of thermal treatment. The introduction of C60 in PCDTBT/PC61 BM blend is here shown to decrease significantly the aggregation of PC61 BM by thermal treatment. This explains the enhanced thermal stability of OSCs.

4 hours

2 hours

8 hours

C60 fraction: 0 500 um

500 um

500 um

C60 fraction: 0.4 500 um

500 um

500 um

500 um

500 um

500 um

C60 fraction: 0.7

Figure 8.21: Microscopic images of PCDTBT/PC61 BM(1−x) /C60(x) films thermally treated at 160 o C for 2 hours, 4 hours and 8 hours (magnification 20).

135

Studies on new generation donor polymers

4 hours

2 hours

8 hours

C60 fraction: 0

C60 fraction: 0.4

200 um

200 um

200 um

200 um

200 um

200 um

Figure 8.22: Microscopic images of PCDTBT/PC61 BM(1−x) /C60(x) films thermally treated at 160 o C for 2 hours, 4 hours and 8 hours (magnification 50).

8.2.5

Conclusions

Polymer chain ordering was controlled using processing additives such as DIO and ODT. However, the introduction of these processing additives is inefficient in increasing the performance of PCDTBT/PC61 BM - OSCs and even decreases it. The use of a ternary blend based on PCDTBT/PC61 BM(1−x) /C60(x) appears to alter the aggregation of PC61 BM and to cause a disruption of PCDTBT polymer chains. Measurements of photovoltaic parameters of ternary blend-OSCs show that the PCE is relatively independent on the C60 fraction as long as the fraction is ≤ 0.7. This result is relevant for the prospect of low cost OSCs because PC61 BM is a relatively expensive material (∼ 10 times more expensive than C60 [2]). Around 10 to 30% of the total fabrication costs of organic solar modules is attributed to the organic semiconductors [13, 150], therefore the replacement of a fraction of PC61 BM by C60 would reduce module costs. Also, the introduction of C60 was shown to prevent the aggregation of PC61 BM upon thermal annealing. As a result, the OSCs fabricated from ternary blends demonstrate significant thermal stability compared to the binary blend.

136

Chapter 9

Conclusions and future work

137

Conclusions and future work

9.1

Conclusions

This thesis provides a comprehensive study on the introduction of processing additives used to increase the performance of solution-processed BHJ-OSCs. First, a systematic method for the selection of processing additives was developed. This method uses the Hansen theory to describe the solubility properties of the organic semiconductors under study and to determine numerical figures of merit that can be used for selecting suitable processing additives. This method was successfully applied to the P3HT/PC61 BM-system. Three novel processing additives, that result in up to a two-fold increase in the PCE, were identified. The mechanistic role of processing additives in improving the BHJ morphology was elucidated by correlating structural and optical characterizations. Studies of the trends in Voc and UV-visible absorption spectra showed that processing additives increased the overall crystallinity in P3HT in the BHJ whereas XRD patterns revealed that the increased crystallinity is associated with a decrease in the polymer crystallite sizes. These results suggest that processing additives lead to the formation of a BHJ with more numerous but smaller polymer crystallites. The photovoltaic performances of additive treated-OSCs were shown to significantly depend on the device architecture. These differences were attributed to variations in hole and electron mobilities induced by the introduction of additives. As the excitons are predominantly generated near the bottom of the active layer, holes and electrons are required to travel different distances before their collection by the respective electrodes. As a result, the photovoltaic performance of OSCs is dependent on the ratio between electron and hole mobilities. Since the mobilities are influenced by the processing additives, the device architecture must be considered when additives are used. As OSC stability is equally important as the PCE, the effects of processing additives on OSC stability were studied by means of photo-stability measurements. Compared to the commonly used thermal annealing technique, the results show that the use of additives improves the photo-stability of OSCs. The best results

138

Conclusions and future work

are obtained with ODT-treated OSCs. Several photo-stability studies, which include adhesion measurements, show that light irradiation causes interfacial degradation. Interestingly, adhesion measurements show that ODT-treated OSCs suffer less from interfacial photo-degradation than thermally annealed OSCs, which could explain the improved life time. Besides P3HT, two other polymers were studied: the semi-crystalline PDQT and the amorphous PCDTBT. In the case of PDQT, ODT and DIO are successful in increasing the PCE of OSCs, indicating that the additive selection rules identified for P3HT/PC61 BM also apply to this case. On the other hand, the additive approach appeared to be unsuccessful for PCDTBT-based OSCs, most likely due to the amorphous character of PCDTBT, which does not require control of the crystallinity as is required for than semi-crystalline polymers. These results suggest that the selection rules designed for P3HT/PC61 BM systems apply for semi-crystalline polymers but not for amorphous polymers.

9.2

Future work

Chapter 8 showed that the use of conventional processing additives failed to improve the performance of PCDTBT-based OSCs. Although additives increased the PCE of PDQT-based OSCs, they appeared to have a different effect on the BHJ morphology than they have on P3HT-based OSCs. Overall, the effectiveness of additives is found to strongly depend on the type of polymer. Therefore, the additive selection method developed in this thesis based on the P3HT/PC61 BM system needs to be extended to a more general perspective. Not only the solubility properties, but several other aspects of the D polymers need to be considered, namely the tendency to crystallize, the molecular weight, the nature of the side chains and the miscibility with A materials. A next step would be to identify how processing additives affect the morphology of BHJ-OSCs as a function of the properties of the D polymers. This is an essential step towards a more general applicability of processing additives and a streamlined way to formulate D/A blends for high efficiency OSCs. The photo-stability tests suggest that additives can be employed to perform a double function: to increase the efficiency and to increase the photo-stability of 139

Appendices

OSCs. To fully exploit this double function, the specific case of ODT-treated OSCs needs to be understood due to their particularly high photo-stability. Two possible hypotheses about the role of additive in this regard arise: either the increase in interfacial adhesion is due to morphological changes in the active layer or it is due to the presence of additive. Although the PM-IRRAS spectra gave no trace of ODT in the BHJ, remaining traces of ODT may be present in quantities that are below the detection limit of this technique. ODT contains two thiol groups which are known for acting as scavengers for free radicals [202], which can be generated during photo-degradation. Therefore, the presence of these scavenger groups may prevent free radical reactions that cause degradation. One idea to test the radical scavenging function of ODT would be to introduce another thiol containing molecule in the active layer instead of ODT and to test the OSC photo-stability. Another stability-related subject raised by this thesis concerns the introduction of C60 in the active layer. The ternary blend based on PCDTBT/PC61 BM(x) /C60(1−x) was shown to exhibit a higher thermal stability compared with the binary system based on PCDTBT/PC61 BM. Replacing a fraction of PC61 BM by C60 is attractive as a method to increase thermal stability in addition to economic benefits. To fully exploit the beneficial properties of C60 as an acceptor, a binary system based on PCDTBT/C60 would be of great interest. However, when a pure PCDTBT/C60 system was made, it exhibited a low PCE, likely due to morphological issues in the BHJ. Processing additives that can help to improve the morphology of the acceptor in the BHJ should be therefore investigated. To date, processing additives are generally investigated for their effects on the donor polymer. Further work should consider the effects on the acceptor in order to optimize systems such as PCDTBT/C60 .

140

Appendix A

Scientific communications Scientific publications U. Vongsaysy, B. Pavageau, G. Wantz, D.M. Bassani, L. Servant, H. Aziz Guiding the selection of processing additives for increasing the efficiency of bulk heterojunction polymeric solar cells. Advanced Energy Materials, 4 (3) 2014 (2013). U. Vongsaysy, D.M. Bassani, L. Servant, B. Pavageau, G. Wantz, H. Aziz Formulation strategies for optimizing the morphology of polymeric bulk heterojunction organic solar cells: a brief review. Journal of Photonics for Energy, 4 (1) (2014). Oral presentations U. Vongsaysy, B. Pavageau, G. Wantz, D.M. Bassani, L. Servant, H. Aziz. Processing Additives for Polymeric Bulk Heterojunction Organic Solar Cells. Young Scientists Symposium Nanorgasol, M`eze, France, 2013. U. Vongsaysy, B. Pavageau, G. Wantz, D.M. Bassani, L. Servant, H. Aziz. Active layer concepts for increased feasibility. IDS-FunMat Training School 2011, Sesimbra, Portugal, 2011. Poster presentations U. Vongsaysy, B. Pavageau, G. Wantz, D.M. Bassani, L. Servant, H. Aziz. Selection of processing additives for polymeric organic solar cells. SPIE Optics + Photonics, San Diego, United States of America, 2014.

141

Scientific communications

U. Vongsaysy, B. Pavageau, G. Wantz, D.M. Bassani, L. Servant, H. Aziz. Processing additives for polymeric organic photovoltaics IDS-FunMat Training School 2013, Annecy, France, 2013. U. Vongsaysy, B. Pavageau, G. Wantz, D.M. Bassani, L. Servant, H. Aziz. Selection of processing additives for polymeric organic photovoltaics Workshop Soochow-University of Waterloo, Waterloo, Canada, 2013. U. Vongsaysy, B. Pavageau, G. Wantz, D.M. Bassani, L. Servant, H. Aziz. Guiding the selection of solvent additives for improving the efficiency of bulk heterojunction polymer solar cells 11th International Symposium on Functional π-electron systems, Arcachon, France, 2013. U. Vongsaysy, B. Pavageau, G. Wantz, D.M. Bassani, L. Servant, H. Aziz.{Guiding the selection of solvent additives for improving the efficiency of bulk heterojunction polymer solar cells The 7ht annual Solvay-Cope symposium on organic electronics, Bordeaux, France, 2013. U. Vongsaysy, B. Pavageau, G. Wantz, D.M. Bassani, L. Servant, H. Aziz. Organic Photovoltaics: New concepts for increased feasibility IDS-FunMat Training School 2012, Anglet, France, 2012. U. Vongsaysy, B. Pavageau, G. Wantz, D.M. Bassani, L. Servant, H. Aziz. Organic Photovoltaics: New concepts for increased feasibility 15 ` eme Journ´ ee de ´ l’Ecole doctorale des sciences chimiques, Bordeaux, France, 2012. U. Vongsaysy, B. Pavageau, G. Wantz, D.M. Bassani, L. Servant, H. Aziz. Organic Photovoltaics: New concepts for increased feasibility Rhodia internal presentation, Pessac, France, 2011.

142

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